feat(otel): metrics support

[mabdinur]

What does this PR do?

Adds OpenTelemetry Metrics SDK support with OTLP transport to datadog-opentelemetry. Metrics can be exported via gRPC (default) or HTTP/protobuf, enabling Rust applications to export OpenTelemetry metrics to Datadog using the standard OpenTelemetry Metrics API.

Key Features

• OTLP Transport: Supports gRPC (default, port 4317) and HTTP/protobuf (port 4318) protocols
• Configuration API: Full support for OTLP metrics configuration via ConfigBuilder, including set_otel_resource_attributes()
• Resource Attributes: Support for OTEL_RESOURCE_ATTRIBUTES environment variable
• Temporality Control: Configurable Delta/Cumulative temporality preference
• Graceful Degradation: HTTP/JSON protocol gracefully degrades with a warning (no-op provider)

Motivation

Enables Rust applications to export OpenTelemetry metrics to Datadog using the standard OpenTelemetry Metrics API, aligning with support in other languages and providing a consistent observability experience across the Datadog ecosystem.

Additional Notes

• Metrics are opt-in via DD_METRICS_OTEL_ENABLED=true

Feature gating:

• Default: cargo add datadog-opentelemetry → gRPC transport
• HTTP-only: cargo add datadog-opentelemetry --no-default-features --features metrics-http → HTTP/protobuf without gRPC dependencies
• Both: cargo add datadog-opentelemetry --features metrics-grpc,metrics-http → enables both protocols

Testing:

• Implementation is fully tested here: otel metrics: add otel parametric endpoints to rust app system-tests#5863

[mabdinur]

Shared tests covering edge cases: DataDog/system-tests#5863