Add HTTP/protobuf OTLP export support for Apache and Nginx modules

[pavolloffay]

This change adds runtime OTLP protocol selection for the otel-webserver-module,
allowing users to choose between grpc, http/protobuf, and http/json exporters.

• Link both libopentelemetry_exporter_otlp_grpc.so and
  libopentelemetry_exporter_otlp_http.so
• Add ApacheModuleOtelExporterOtlpProtocol and
  NginxModuleOtelExporterOtlpProtocol configuration directives
• Support OTEL_EXPORTER_OTLP_TRACES_PROTOCOL and OTEL_EXPORTER_OTLP_PROTOCOL
  environment variable fallbacks
• Default to http/protobuf per OTel specification

Fixes #614

[pavolloffay]

@chrlic could you please review?

[chrlic]

It looks good to me, very useful addition.

[lalitb]

Thanks @pavolloffay for the PR -

Before merging this, can we fix/refresh the webserver CI and dependency versions? This PR changes code that should be compiled against the webserver module’s pinned dependencies, but the current CI does not appear reliable because some of the runner/distro setup is outdated. At the same time, the module still advertises/builds against older pinned versions, including opentelemetry-cpp 1.2.0.

It would be safer to first update the CI to a supported runner/distro setup and make sure the module builds against the intended OpenTelemetry C++ SDK version, then validate this PR against that. Otherwise we may merge API usage or runtime behavior that is not actually compatible with the versions this module packages.

Also as mentioned in slack - the modules is currently not-maintained, and we are looking for the maintainers for this.