[APM] Verify that UI is working as expected after _doc_count is added to transactions/spans

[dgieselaar]

There currently is an in-flight PR to set _doc_count for transactions and spans, so results on sampled data are automatically upscaled.

The expectation is that this should need no work from the UI, but we should verify. In summary, the changes are:

• Any bucketing aggregation will use _doc_count and opaquely upscale results
• track_total_hits will still use the real document count

We should check the following:

• Are we using track_total_hits somewhere, where we actually need the upscaled doc count?
• Are we using bucketing aggregations somewhere, where we actually need the real doc count?

Ideally we should also have some synthtrace tests. We currently have sampled(), but I suggest we replace that with count(n), which would set both {transaction,span}.representative_count and _doc_count to n.

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)

[yngrdyn]

Following yesterday discussion on how to show to the user something has changed in the Based on sample transactions tooltip

we could use a popover that component would allow us to place a link to documentation, the only thing is the visual styling is different from the tooltip we display now.