[Feature Request] Add Official Metrics Support to OTLP Exporters

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

Start by reading the HTTP and gRPC OTLP exporter implementations and the official OpenTelemetry Metrics SDK integration points. Define how optional metrics avoid circular dependencies, then apply the message-size, request-duration, and export-failure measurements consistently to both exporters. Done means configurable metrics with the specified attributes and units, without relying on MetricsReporter.