Set agent name and version conditionally

[gregkalapos]

agent.name is a required field in Kibana - without that the APM UI throws an exception.

Via the elasticapmintake receiver these fields are set and we get the values out of the box (elastic APM Agents send those values) - and the processor just disabled these 2 fields: https://github.com/gregkalapos/opentelemetry-collector-components/blob/main/processor/elasticapmprocessor/processor.go#L69-L70

For OTel SDKs via the OTLP receiver, we just called the s.setAgentName and s.setAgentVersion functions to calculate those - because the lines below for disabling the fields are only active for ecs mode.

This breaks when OTel SDKs are used in ecs mode ("internal linked" to internal issue below for more details).

The idea:

• in the opentelemetry-lib here we calculate these 2 fields when they are not present - otherwise just take them as-is.
• I'll open another issue to remove the p.enricher.Config.Resource.Agent[Name|version].Enabled = false lines for ecs mode - update: PR in the other repo: [elasticapmprocessor] - do not disable AgentName and AgentVersion in ECS mode opentelemetry-collector-components#758

This way, we handle it like this:

• Elastic APM Agents via elasticapmintake receiver: values are present, just take them as is
• OTLP receiver with otel mode: value not present, we calculate it
• OTLP receiver with ecs mode: same as above

[girodav]

LGTM