[receiver/datadogreceiver] add json handling for the api/v2/series endpoint

[jdgeisler]

Description

Adding json handling for the api/v2/series endpoint. The datadog api client libraries use json messages, however only protobuf messages are currently supported in the api/v2/series endpoint, so requests fail with proto: illegal wireType 6

If Content-Type: application/json is set, then we handle the json message. Otherwise, we handle the protobuf message.

Link to tracking issue

Fixes #36079

Testing

Added test with a json metric payload that now passes.

Additionally, I also tested these changes in my own image and confirmed that the datadog api client libraries can now successfully ship metrics to the api/v2/series endpoint.

I also confirmed with the following curl:

curl -X POST \
  -H "Content-Type: application/json" \
  -H "DD-API-KEY: your_api_key_here" \
  -d '{
    "series": [
      {
        "resources": [
          {
            "name": "dummyhost",
            "type": "host"
          }
        ],
        "tags": ["env:test"],
        "metric": "test.metric",
        "points": [
          {
            "timestamp": 1730829575,
            "value": 1.0
          }
        ],
        "type": 3
      }
    ]
  }' \
  https://datadog-receiver/api/v2/series
{"errors":[]}