Commit Message: access_log: add OTLP/HTTP exporter to OpenTelemetry access logger

Additional Description: Adds HTTP transport support to the OpenTelemetry access logger, complementing #29207 (OTLP/HTTP tracing). This enables direct OTLP log export to backends that only accept HTTP (Dynatrace, Datadog, Logfire), without requiring an intermediate collector sidecar.

Also refactors the config to match the tracer pattern:

• Add top-level http_service field for OTLP/HTTP transport
• Add top-level grpc_service and log_name fields
• Deprecate common_config field (still functional for backward compatibility)
• Extract shared utilities (otlp_log_utils) for HTTP and gRPC implementations

Risk Level: Low (new optional feature, backward compatible)

Testing: Unit tests, integration test, manual testing with OTel collector

Docs Changes: Proto documentation updated

Release Notes: Added

Fixes #26352 (access logs portion)

Example Configuration (OTLP/HTTP with resource_attributes)

access_log:
- name: envoy.access_loggers.open_telemetry
  typed_config:
    "@type": type.googleapis.com/envoy.extensions.access_loggers.open_telemetry.v3.OpenTelemetryAccessLogConfig
    http_service:
      http_uri:
        uri: "http://localhost:4318/v1/logs"
        cluster: otel_collector
        timeout: 1s
    log_name: envoy_access_log
    resource_attributes:
      values:
      - key: "service.name"
        value:
          string_value: "my-service"
    body:
      string_value: "%REQ(:METHOD)% %REQ(:PATH)% %RESPONSE_CODE%"

Example Configuration (OTLP/gRPC - top-level field)

access_log:
- name: envoy.access_loggers.open_telemetry
  typed_config:
    "@type": type.googleapis.com/envoy.extensions.access_loggers.open_telemetry.v3.OpenTelemetryAccessLogConfig
    grpc_service:
      envoy_grpc:
        cluster_name: otel_collector
    log_name: envoy_access_log
    body:
      string_value: "%REQ(:METHOD)% %REQ(:PATH)% %RESPONSE_CODE%"

---

Screenshots

Request variables

Key	Value
ref	6314ffc
sha	2143126
pr	42445
base-sha	ac2b565
actor	@codefromthecrypt
message	access_log: add OTLP/HTTP exporter to OpenTelemetry access logger...
started	1767909464.751212
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)