Add HTTP/protobuf OTLP export support to otel-webserver-module

Summary

Add support for HTTP/protobuf OTLP export protocol alongside the existing gRPC export in the otel-webserver-module (used for Apache HTTPD and Nginx auto-instrumentation).

Motivation

The OpenTelemetry Operator provides auto-instrumentation for multiple languages (Java, Python, Node.js, .NET, Go). All other language auto-instrumentations support configuring the OTLP export protocol via the OTEL_EXPORTER_OTLP_PROTOCOL environment variable, and default to http/protobuf. The Apache HTTPD and Nginx auto-instrumentations are the exception — they hardcode gRPC at the native C++ level by directly linking libopentelemetry_exporter_otlp_grpc.so.

This creates operational friction:

• Users must ensure their collector exposes the gRPC endpoint (port 4317) specifically for these two instrumentations, even if everything else in their environment uses HTTP/protobuf (port 4318)
• The OTEL_EXPORTER_OTLP_PROTOCOL environment variable is silently ignored, leading to confusion
• Operator users cannot use a uniform collector configuration across all auto-instrumented workloads

Proposed Change

1. Link both OTLP exporter libraries — Build the webserver module against both opentelemetry_exporter_otlp_grpc and opentelemetry_exporter_otlp_http
2. Add runtime protocol selection — Read OTEL_EXPORTER_OTLP_PROTOCOL (and the signal-specific variants OTEL_EXPORTER_OTLP_TRACES_PROTOCOL, etc.) at exporter initialization time
3. Use the appropriate factory — Instantiate OtlpHttpExporterFactory or OtlpGrpcExporterFactory based on the configured protocol
4. Default to http/protobuf — Align with the OpenTelemetry specification default and the behavior of all other auto-instrumentations

This is architecturally straightforward because the upstream opentelemetry-cpp SDK already provides both exporters behind a shared SpanExporter interface. The webserver module pipeline is transport-agnostic — only the exporter instantiation point needs to change.

Scope

• Apache HTTPD auto-instrumentation
• Nginx auto-instrumentation
• CMake/build changes to link the HTTP exporter library and its dependency (libcurl)
• Supported protocol values: grpc, http/protobuf, http/json

Related

• Discussion on separating OTLP proto from transport: https://github.com/open-telemetry/opentelemetry-cpp-contrib/discussions/410
• OpenTelemetry specification on OTEL_EXPORTER_OTLP_PROTOCOL: https://opentelemetry.io/docs/specs/otel/protocol/exporter/

Start by locating the otel-webserver-module exporter initialization and its CMake targets for Apache HTTPD and Nginx. Check how OTEL_EXPORTER_OTLP_PROTOCOL and its signal-specific variants should select the grpc, http/protobuf, or http/json factory, then verify both modules build with the HTTP exporter and default to http/protobuf.