Envoy Json-To-Metadata using Array to match json body

[rlanhellas]

Title: Improvement to accept array match on Json-To-Metadata filter

Description:
I'm using the Json-To-Metadata filter and it works very well for JSON fields outside arrays. But Imagine the following structure:

{
 "data": {
    "regions": [
        {"name": "america"}
      ]
 }
}

How can I match using data.regions[0] where value equal to america ? Simple answer. I can't. Because the current filter is using node->getObject() (https://github.com/envoyproxy/envoy/blob/main/source/extensions/filters/http/json_to_metadata/filter.cc#L370), it never expects an array.

My suggestion would be to enable the support to array, at least for a specific position in array, we don't need to iterate over all array, just get an index and match the value. I implemented that solution locally and worked perfectly, so here is my idea:

absl::StatusOr<Json::ObjectSharedPtr> next_node_result;
const std::string keyword_array = "@array_";
const std::int16_t keyword_array_sizeof = 7;
for (unsigned long i = 0; i < keys.size() - 1; i++) {
  if (keys[i].find(keyword_array) != std::string::nops){
    next_node_result = node->getObjectArray(keys[i].substr(keyword_array_sizeof))[0];
  }else{
     next_node_result = node->getObject(keys[i]);
  }
}

A bit explanation about my idea. If the selector has @array_ it means we want to use array and not just simple object. In that case we use the getObjectArray and not getObject. Now the selector would be data.@array_regions.

In this sample code I'm getting always the first item of array (index zero), but we should add it to selector as well, like this one: data.@array_3_regions, so we can pick up the correct item.

For me lookup through an array is very important and a requirement, so we can't use this filter without this. I have the code ready to open a PR but would like to hear you guys first and I can push my PR.