[exporter/loadbalancing] allow metrics routing

[claudiobastos]

Closes #25858

Description: Add metrics exporter that will balance its metrics considering new routing choices

Link to tracking Issue: #25858

Testing: Add tests for new routing choices and metrics exporter

[linux-foundation-easycla]

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• ✅ login: claudiobastos / name: Cláudio Bastos (f805ff7, 50dfb8b, 737767b, f89fd48, 4ee2e3b, d50b84d, 6e9ecf8, f1eedc1, db0a79a, 58ce078, 7fe4027, 9eb8b09, 01c4f7c, ba9e5f6, 42fa8e8, 6fb23c2, b9edce8, 5027b3e, 1a93002, 8fa9df9)

[claudiobastos]

In this second iteration I've tought about it and decide that for metrics, if routing key is empty or set to traceId, metrics exporter will consider service's.

[jpkrohling]

In this second iteration I've tought about it and decide that for metrics, if routing key is empty or set to traceId, metrics exporter will consider service's.

If it's set to traceID, it should return an error: the user has explicitly stated something that cannot be honored. We also have a

[jpkrohling]

Sorry that it took so long, but this LGTM. I did some basic manual testing as well, as follows:

Config used:

receivers:
  otlp/loadbalancer:
    protocols:
      grpc:
      http:

  otlp/backend-1:
    protocols:
      grpc:
        endpoint: localhost:5317
  otlp/backend-2:
    protocols:
      grpc:
        endpoint: localhost:6317
  otlp/backend-3:
    protocols:
      grpc:
        endpoint: localhost:7317

processors:

exporters:
  logging/loadbalancer:
  logging/backend-1:
  logging/backend-2:
  logging/backend-3:

  loadbalancing:
    routing_key: "service"
    protocol:
      otlp:
        tls:
          insecure: true
    resolver:
      static:
        hostnames:
        - localhost:5317
        - localhost:6317
        - localhost:7317

service:
  pipelines:
    metrics/loadbalancer:
      receivers:
        - otlp/loadbalancer
      processors: []
      exporters:
        - logging/loadbalancer
        - loadbalancing

    metrics/backend-1:
      receivers:
        - otlp/backend-1
      processors: []
      exporters:
        - logging/backend-1
    metrics/backend-2:
      receivers:
        - otlp/backend-2
      processors: []
      exporters:
        - logging/backend-2
    metrics/backend-3:
      receivers:
        - otlp/backend-3
      processors: []
      exporters:
        - logging/backend-3

Commands used to test:

> telemetrygen metrics --otlp-insecure --otlp-attributes='service.name="foo"' --metrics 100
> telemetrygen metrics --otlp-insecure --otlp-attributes='service.name="bar"' --metrics 100
> telemetrygen metrics --otlp-insecure --otlp-attributes='service.name="acme"' --metrics 100

Relevant metrics:

otelcol_exporter_sent_metric_points{exporter="logging/backend-1",service_instance_id="bbd1a465-b152-40ff-9a0f-281bad72c957",service_name="otelcontribcol",service_version="0.84.0-dev"} 100
otelcol_exporter_sent_metric_points{exporter="logging/backend-2",service_instance_id="bbd1a465-b152-40ff-9a0f-281bad72c957",service_name="otelcontribcol",service_version="0.84.0-dev"} 100
otelcol_exporter_sent_metric_points{exporter="logging/backend-3",service_instance_id="bbd1a465-b152-40ff-9a0f-281bad72c957",service_name="otelcontribcol",service_version="0.84.0-dev"} 100
otelcol_exporter_sent_metric_points{exporter="logging/loadbalancer",service_instance_id="bbd1a465-b152-40ff-9a0f-281bad72c957",service_name="otelcontribcol",service_version="0.84.0-dev"} 300