Differentiate between service-level labels and user-specific labels

[dgieselaar]

Currently, any label added via addLabels() is treated as a "global" label. Global labels are part of the aggregation key of (service) transaction metrics, meaning that the user risks creating a high amount of metric groups that will result in APM Server pushing data into an overflow bucket. This can happen if user-specific labels or metrics are added via the addLabels() API. For 8.7.1 we are removing global labels from the aggregation key for the RUM agent (elastic/apm-server#10532). Beyond that, we need a better fix that gives the user the option to aggregate on service-specific labels. I'm not sure what exactly, but perhaps we can add a new addGlobalLabels() API, not sure if that is the best option though.

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Additional context: https://elastic.slack.com/archives/C5L79P420/p1679555445014589

[paulb-elastic]

Discussed with @felixbarny and will work on this in the future when we plan in a bigger scoped breaking change in the RUM agent. The APM Server work around will continue to be the mitigation for the time being.