[connector/elasticapm] Add `data_stream.namespace` attribute as resource attribute for aggregated metrics

[lahsivjar]

Aggregated metrics currently don't aggregate on data_stream.namespace attribute which is used in routing to achieve logical isolation within Elasticsearch. The workaround for this involves adding custom resource attributes to the elastic apm connector but given the usefulness of this metric, it makes sense to add it globally.

• Update the configuration to add data_stream.namespace as a resource attribute.
• Validate existing tests in collector-components and other mOTLP codebases.
• Check for any changes required in the elastic-agent repository.
• Release changes to mOTLP
• Release changes to elastic-agent.

See: https://elastic.slack.com/archives/C031284ANNL/p1770727291228679

[lahsivjar]

Release to elastic-agent is blocked by elastic/elastic-agent#10156