[Telemetry] 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

• [META 896] Set User-Agent in OTLP exporters elastic-otel-java#448
• [META 896] Set User-Agent in OTLP exporters elastic-otel-dotnet#171
• [META 896] Set User-Agent in OTLP exporters elastic-otel-node#431
• [META 896] Set User-Agent in OTLP exporters elastic-otel-python#172
• [META 896] Set User-Agent in OTLP exporters elastic-otel-php#118