[connector/elasticapm] Allow configuration of custom dimensions for all aggregated metrics.

[lahsivjar]

Motivation

The dimensions are pre-configured for the signaltometrics component used within the elasticapm connector. This is a bit restrictive to some use-cases where users want to customize the dimensions produced by the aggregations. Customizing dimensions makes the UI for flexible as users can then search the APM UI with the custom aggregated dimensions.

Implementation

Expose configuration in the elasticapm connector to customize the aggregated metrics. Note that the configs offered in the elasticapm connector should use the abstractions of service_summary, service_trsansactoin, service_destination, and transaction instead of the more flexible options offered by the signal to metrics component.