[exporter/datadog] Report otelcol_datadog_otlp_translator_resources_missing_source metric

[mx-psi]

Description:

Adds new telemetry metric, otelcol_datadog_otlp_translator_resources_missing_source, that counts OTLP resources missing a hostname-identifying attribute.

Datadog correlation and infrastructure features may not work correctly or be fully featured if payloads are missing a hostname; this metric is useful to track this.

Internal reference: OTEL-1155

Testing: Tested with the following configuration:

Configuration (Click to expand)

receivers:
  prometheus:
    config:
      scrape_configs:
      - job_name: 'otelcol'
        scrape_interval: 10s
        static_configs:
        - targets: ['0.0.0.0:8888']
        metric_relabel_configs:
          - source_labels: [ __name__ ]
            regex: '.*grpc_io.*'
            action: drop
    
  hostmetrics:
    scrapers:
      load:

exporters:
  datadog:
    api::key: ${env:DD_API_KEY}
  debug:

service:
  pipelines:
    metrics:
      receivers: [prometheus, hostmetrics]
      exporters: [debug, datadog]

Documentation: To be added on Datadog docs