Support global labels for aggregated metrics for RUM

Global labels can have high cardinality when used for RUM. They are not currently part of aggregated metrics since https://github.com/elastic/apm-server/pull/10624, to avoid causes for `overflow` buckets. This currently impacts apm data collected by the "rum-js", "js-base", "android/java", "iOS/swift" agents.

We should find a way to also support global labels for RUM.

Update the docs once this is completed, to undo the change in https://github.com/elastic/observability-docs/pull/3877

Start by reviewing the change in apm-server PR 10624 and the documentation change in observability-docs PR 3877 to understand the overflow-bucket constraint and the current RUM behavior. Determine a safe design for including global labels in aggregated RUM metrics, then update the documentation to undo the documented limitation and verify the affected rum-js, js-base, android/java, and iOS/swift agent cases.

Written by the indexing model from the issue text.