Set User-Agent in OTLP exporters

[JonasKunz]

Description

For telemetry purposes we want to unify the User-Agent headers sent by the OTLP exporters used by our distros. We should configure the User-Agent header there to the following format (or similar): elastic-otlp-(grpc/http)-<language>/<edot-version>

Agent Issues

• [ ] https://github.com/elastic/elastic-otel-java/issues/448
• [ ] https://github.com/elastic/elastic-otel-dotnet/issues/171
• [ ] https://github.com/elastic/elastic-otel-node/issues/431
• [ ] https://github.com/elastic/elastic-otel-python/issues/172
• [ ] https://github.com/elastic/elastic-otel-php/issues/118