OpenTelemetry extension should set service.name resource attribute

[jmacd]

Title: Set the OpenTelemetry service.name resource attribute

Description:
OpenTelemetry specifies semantic conventions for services to meet, the most basic being the service's name.

Repro steps:
First reported here: #21370 (comment)

The Envoy should emit well-formed OTLP span data. The OpenTelemetry conventions require a service.name resource attribute, so one should be set either using configuration or a default value. OpenTelemetry specifies how to pick a default value, for example:

[1]: MUST be the same for all instances of horizontally scaled services. If the value was not specified, SDKs MUST fallback to unknown_service: concatenated with process.executable.name, e.g. unknown_service:bash. If process.executable.name is not available, the value MUST be set to unknown_service.

[moderation]

Looking at some traces being sent by the new Otel tracer, it looks like it is adhering to the spec but it would be nice to have additional information sent here from the Envoy config

[AlexanderEllis]

Good catch! I think this should be relatively simply to set up and pipe through — I'm imagining a new optional field in the OpenTelemetryConfig proto, then checking for it or defaulting to unknown_service (or maybe unknown_service:envoy, which seems like a sensible default in this case) when building the ResourceSpans proto during export.

[phlax]

@AlexanderEllis i have assigned this to you - i hope that is ok