docs: add migration guide

packages/opentelemetry-node/docs/migration-from-apm-agent.md

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+<!--
+Goal of this doc:
+Provide a reference of how to switch from `elastic-apm-node`, the classic agent, to `@elastic/opentelemetry-node`. It also
+provide guide on passing their ELASTIC_* instrumentation config to EDOT
+
+Assumptions we're comfortable making about the reader:
+* They are just starting the agent in any form (REVIEW)
+* They are not using any apm, transaction or span APIs (REVIEW)
+
+-->
+
+# Migration from Elastic APM Node.js Agent
+
+This guide shows you how to _migrate_ a service instrumented with Elastic APM Node.js Agent
+(classic Agent) with a basic setup to Elastic Distribution of OpenTelemetry Node.js (EDOT Node.js). If your
+application is not instrumented with the classic agent please visit
+our [get started](./get-started.md) guide in this documentation.
+
+
+<!-- ✅ What the user needs to know and/or do before they migrate EDOT Node.js -->
+## Prerequisites
+
+Before getting started, you'll need to save a minimal set of configuration options from the classic Agent, so
+EDOT Node.js can send data to the same deployment of [Elastic Observability](https://www.elastic.co/observability) using the same service name.
+
+The mininmal set of options required for the migration are:
+
+- [server url](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/configuration.html#server-url)
+- [api key](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/configuration.html#api-key) or [secret token](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/configuration.html#secret-token)
+- [service name](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/configuration.html#service-name) (optional)
+
+
+
+<!-- ✅ Step-by-step instructions of the most common scenario -->
+## Migration process
+
+If the service being instrumented is not using any advanced feature like [Agent's APIs](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/api.html) the
+migration process is straight-forward. This section will focus on this scenario and instructions for advanced usage
+will be given in futeher sections.
+
+### Uninstall Elastic APM Node.js Agent
+
+Uninstall the `elastic-apm-node` package:
+
+```sh
+npm uninstall --save elastic-apm-node
+```
+
+Once the pacakge is removed you should proceed to remove any setup made to start the classic agent along with your service.
+
+- remove Node.js `--require` CLI option `NODE_OPTIONS` or the start command if your service is using [Node.js CLI option](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/starting-the-agent.html#start-option-node-require-opt) start method.
+- remove start snippet if your service is starting the agent with one of this options
+  * [require('elastic-apm-node').start(...)](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/starting-the-agent.html#start-option-require-and-start)
+  * [require('elastic-apm-node/start')](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/starting-the-agent.html#start-option-require-start-module)
+- remove the APM module if your service is using [separate APM init module](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/starting-the-agent.html#start-option-separate-init-module)
+
+### 2. Install EDOT Node.js
+
+Install the `@elastic/opentelemetry-node` package:
+
+```sh
+npm install --save @elastic/opentelemetry-node
+```
+
+Like Elastic APM Node.js Agent EDOT Node.js is a single package that includes all the
+OpenTelemetry JS packages that are needed for most cases.
+
+Once you have the package installed is time to setup the start proces. The recommended way 
+of starting EDOT Node.js along your application is via `--require` CLI option.
+
+```sh
+node --require @elastic/opentelemetry-node my-service.js
+```
+
+<!-- ✅ How to change the configuration -->
+## Configuration of EDOT Node.js
+
+Once you're done with the changes described in the section above it is time to migrate the configuration. EDOT Node.js is
+typically configured with `OTEL_*` environment variables defined by the OpenTelemetry spec. Environment variables
+are read at startup and can be used to configure EDOT Node.js.
+
+For the minimal configuration mentioned in [prerequisites](#prerequisites) section there are `OTEL_*` variables
+from OpenTelemetry spec that fit.
+
+- `OTEL_EXPORTER_OTLP_ENDPOINT` will hold the value of `server_url` configuration option
+- `OTEL_EXPORTER_OTLP_HEADERS` will hold the value of `api_key` or `secret_token`
+  * the value should be `Authorization=Bearer {secret_token}` if `secret_token` was used
+  * the value should be `Authorization=ApiKEy {api_key}` if `api_key` was used
+
+If you want more information about configuration you may visit [configuration options](./configure.md) page.
+
+
+
+## Advanced
+
+There might be some cases where the instrumentation 
+
+### Using classic Agent's APIs
+
+If you are doing manual instrumentation of some modules of your service through the [classic Agent's API](https://www.elastic.co/guide/en/apm/agent/nodejs/4.x/api.html)
+you will need to refactor it to use `@opentelemetry/api` methods.
+
+Either if you're using `apm.startTransaction` or `apm.startSpan` APIs both could be refactor to the same instrumentation
+code using `@opentelemetry/api`. The anatomy of a manual instrumentation using the classig Agent consists in
+using `apm.startTransaction` or `apm.startSpan` to mark the beginning of a transaction/span and mark the end
+by calling the `.end` API of the transaction or spans returned before.
+
+Example:
+```js
+// file: my-service.js
+//
+// NOTE: we're assuming this app is started using Node.js --require CLI option
+//   node --require elastic-apm-agent my-service.js
+const http = require('http');
+const apm = require('elastic-apm-agent');
+
+const server = http.createServer(function onRequest(req, res) {
+    // Start of manual instrumentation
+    const span = apm.startSpan('Custom Span')
+
+    console.log('incoming request: %s %s %s', req.method, req.url, req.headers);
+    req.resume();
+    req.on('end', function () {
+        const body = 'pong';
+        res.writeHead(200, {
+            'content-type': 'text/plain',
+            'content-length': Buffer.byteLength(body),
+        });
+        res.end(body);
+        // Instrumentation end
+        span.end();
+    });
+});
+
+server.listen(3000, '127.0.0.1', function () {
+    console.log('listening at', server.address());
+});
+```
+
+To migrate this code to use `@opentelemetry/api` instead you should:
+
+- remove the require call of `elastic-apm-node`
+- require `@opentelemetry/api` and get a tracer
+- replace usage of the classic Agent's API with call to the tracer API
+
+```js
+// file: my-service.js
+//
+// NOTE: we're assuming this app is started using Node.js --require CLI option
+//   node --require @elastic/otel-node my-service.js
+
+const http = require('http');
+// Require OTEL API
+const api = require('@opentelemetry/api');
+// Get a tracer for our manual instrumentation
+const tracer = api.tracer.getTracer();
+
+const server = http.createServer(function onRequest(req, res) {
+    // Start of manual instrumentation. In OTEL is always starting an Span
+    tracer.startActiveSpan('Custom Span', function (span) {  // <-- the measured logic should be wrapped in this callback
+      console.log('incoming request: %s %s %s', req.method, req.url, req.headers);
+      req.resume();
+      req.on('end', function () {
+          const body = 'pong';
+          res.writeHead(200, {
+              'content-type': 'text/plain',
+              'content-length': Buffer.byteLength(body),
+          });
+          res.end(body);
+          // Instrumentation end
+          span.end();
+      });
+    });
+});
+
+server.listen(3000, '127.0.0.1', function () {
+    console.log('listening at', server.address());
+});
+```
+
+
+### Extensive use of config options
+
+TODO: review classic agent config options and add here the ones that can be migrated to OTEL env vars
+
+
+## FAQ
+
+- Will I get the same traces and metrics once the servie is migrated to EDOT Node.js?
+
+TODO: point to supported technologies and metrics documents
+
+- ????