Make mapping explosion on tags visible

[simitt]

Investigate what is the best option to raise awareness around mapping explosion:

• Add information to monitoring tab, by showing number of dropped events and storing APM Server logs or storing documents that could not be indexed.
• Return response to agents for the next request, if documents have been dropped.
• Add a warning to UI if number of fields in and index gets critical
• ..

[alvarolobato]

This issue won't be needed if we finally do this: #1477

[axw]

There's some new discussions ongoing about having labels use the flattened field type, which would eliminate the mapping explosion issue altogether. We will need to address numerical labels somehow (see #3873), and for that we will need elastic/elasticsearch#61550. For flattened labels we will need elastic/kibana#25820.

I'll leave this open for now until there's some more certainty.

[simitt]

We can follow the progress in #5963. Closing this issue for now.