[Config] Add Global Labels

[jackshirazi]

Classic agent link

• https://www.elastic.co/guide/en/apm/agent/java/current/config-core.html#config-global-labels

What it does

Labels added to all events, with the format key=value[,key=value[,...]]. Any labels set by application via the API will override global labels with the same keys. The server sees labels.key=value attributes

Implementation difficulty, location, and dynamism

• This can be implemented as an Exporter (it can be added very easily to ElasticSpanExporter). It could also be implemented as a SpanProcessor eg https://github.com/open-telemetry/opentelemetry-java-instrumentation/discussions/2163 but for performance, especially if it's dynamic, I think it's better in the Exporter
• It can be contributed upstream (but probably not worth it)
• It can be dynamic

[felixbarny]

Isn't OTEL_RESOURCE_ATTRIBUTES the equivalent for this in the OTel world?

[jackshirazi]

Yes, you're right. @akhileshpok please can you feedback on whether this needs to be a dynamically alterable capability, or if the static already existing otel implementation is sufficient

[felixbarny]

For reference, global_labels isn't a dynamic config option, either.

[SylvainJuge]

Another benefit of using otel resource attributes for implementation is that it would allow to provide a cross-platform compatibility, we just have to formally have a specification on the resource attribute key and properly map it into our data model on ingest.