[exporter/stefexporter] Add basic STEF exporter implementation

#### Description

Added STEF exporter implementation for metrics, sending data over gRPC stream.
For now only queuing and retry exporter helpers are used. We will need to
decide later if other helpers are needed for this exporter.

#### Testing

Unit tests that verify connecting, reconnecting, sending, acking of data
are included.

#### Documentation

Added to README.

#### Future Work

More extensive test coverage is desirable and will likely be added
in the future.

We likely want to implement STEF receiver and add STEF as a tested protocol
to our testbed.

exporter/stefexporter/README.md

@@ -13,3 +13,46 @@
 
 Export data via gRPC using
 [Otel/STEF format](https://github.com/splunk/stef/tree/main/go/otel) format.
+
+## Getting Started
+
+The following settings are required:
+
+- `endpoint` (no default): host:port to which the exporter is going to send STEF metric data,
+  using the STEF/gRPC protocol. The valid syntax is described
+  [here](https://github.com/grpc/grpc/blob/master/doc/naming.md).
+  If a scheme of `https` is used then client transport security is enabled and overrides the `insecure` setting.
+- `tls`: see [TLS Configuration Settings](../../config/configtls/README.md) for the full set of available options.
+
+Example:
+
+```yaml
+exporters:
+  stef:
+    endpoint: otelcol2:4317
+    tls:
+      cert_file: file.cert
+      key_file: file.key
+  stef/2:
+    endpoint: otelcol2:4317
+    tls:
+      insecure: true
+```
+
+By default, no compression is enabled. The only supported compression method is zstd.
+To enable, configure as follows:
+
+```yaml
+exporters:
+  otlp:
+    ...
+    compression: zstd
+```
+
+## Advanced Configuration
+
+Several helper files are leveraged to provide additional capabilities automatically:
+
+- [gRPC settings](https://github.com/open-telemetry/opentelemetry-collector/blob/main/config/configgrpc/README.md)
+- [TLS and mTLS settings](https://github.com/open-telemetry/opentelemetry-collector/blob/main/config/configtls/README.md)
+- [Queuing and retry settings](https://github.com/open-telemetry/opentelemetry-collector/blob/main/exporter/exporterhelper/README.md)

exporter/stefexporter/config.go

@@ -10,14 +10,23 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"time"
 
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/config/configgrpc"
+	"go.opentelemetry.io/collector/config/configretry"
+	"go.opentelemetry.io/collector/exporter/exporterhelper"
 )
 
 // Config defines configuration for logging exporter.
 type Config struct {
-	configgrpc.ClientConfig `mapstructure:",squash"` // squash ensures fields are correctly decoded in embedded struct.
+	exporterhelper.QueueConfig `mapstructure:"sending_queue"`
+	RetryConfig                configretry.BackOffConfig `mapstructure:"retry_on_failure"`
+	configgrpc.ClientConfig    `mapstructure:",squash"`
+
+	// For testing purposes we shorten this time to make tests faster.
+	// TODO: decide if this needs to be end-user configurable too.
+	maxAckWaitTime time.Duration
 }
 
 var _ component.Config = (*Config)(nil)
@@ -38,6 +47,13 @@ func (c *Config) Validate() error {
 		return fmt.Errorf(`invalid port "%s"`, port)
 	}
 
+	switch c.Compression {
+	case "":
+	case "zstd":
+	default:
+		return fmt.Errorf("unsupported compression method %q", c.Compression)
+	}
+
 	return nil
 }