Service environment correlation

[SylvainJuge]

Description

Agent implementation of specification change #766

Summary of the required changes:

• updating documentation in https://github.com/elastic/observability-docs/pull/2731 to invite users to provide values for service.version and service.environment (by using SERVICE_NAME and ENVIRONMENT configurations respectively).
• when ecs-logging library is used:
  • agents provide fallback values if they are not set in ecs-logging configuration for: service.name, service.version and service.environment. Giving priority to existing values (if any) avoids making this a breaking change.
  • (optional but highly recommended) if values from agent configuration and ecs-logging configuration differ, issuing a warning is relevant to inform end user that traces and logs can't be properly correlated.
• when log reformatting (and log sending) is used:
  • produced ECS logs should contain service.name, service.version and service.environment with values from agent configuration.

Agent Issues

• [x] https://github.com/elastic/apm-agent-java/issues/3051
• [x] https://github.com/elastic/apm-agent-dotnet/issues/2033
• [x] https://github.com/elastic/apm-agent-nodejs/issues/3195
• [x] https://github.com/elastic/apm-agent-python/issues/1772
• [ ] https://github.com/elastic/apm-agent-go/issues/1401
• [ ] https://github.com/elastic/apm-agent-php/issues/879
• [ ] https://github.com/elastic/apm-agent-ruby/issues/1374