[Feature Request] Add Official Metrics Support to OTLP Exporters

[arjun-rajappa]

Add official OpenTelemetry Metrics API support to OTLP exporters to monitor export performance and reliability.

Metrics Needed

1. Message Sizes

• otel.otlp_exporter.message.uncompressed_size (bytes)
• otel.otlp_exporter.message.compressed_size (bytes, when compression enabled)

2. Request Duration

• otel.otlp_exporter.request.duration (milliseconds)
  • Attribute: http.status_code

3. Export Failures

• otel.otlp_exporter.export.failure (counter)
  • Attribute: error.type (e.g., "timeout", "http_503", "socket_error")

Requirements

• Use official OpenTelemetry Metrics SDK (not the un-spec'd MetricsReporter)
• Make metrics optional/configurable
• Avoid circular dependencies in self-instrumentation
• Apply consistently across HTTP and gRPC exporters

Benefits

• Monitor export pipeline health in production
• Troubleshoot performance issues and failures
• Optimize compression and batch configurations