Support global labels for aggregated metrics for RUM

[simitt]

Global labels can have high cardinality when used for RUM. They are not currently part of aggregated metrics since #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 elastic/observability-docs#3877

[b2ronn]

In version 8.10, global metrics for aggregated indices have still not been introduced.
#11768

[simitt]

@b2ronn due to an often expected high cardinality in RUM global labels, we would need to find a different solution than just adding global labels as an aggregation dimension. This can also be a challenge for non-RUM setups, and we are thinking about a more global solution to this. But the task is not currently scheduled for implementation and will likely not be part of the next minor versions.

[b2ronn]

As an alternative, it may be worth to consider adding the capability for the admin of the APM server to define the list of permitted global labels for "rum-js," "js-base," "android/java," "iOS/swift"?