Add a _doc_count field for single-histogram metricsets

[axw]

Motivation/summary

If a metricset is published with a single histogram, add a _doc_count field which is the sum of the histogram's counts. This will enable us to remove some value_count aggregations in the UI to determine the transaction count.

Checklist

• Update CHANGELOG.asciidoc

How to test these changes

1. Enable transaction metrics aggregation
2. Send some transactions
3. Check that the docs have _doc_count in them
4. Perform aggregations over the docs (e.g. terms agg on transaction.name), and check that the doc_count value in each bucket matches the number of transaction events aggregated.

Related issues

Closes #4474

[apmmachine]

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Build stats

• Build Cause: Pull request #4647 updated

• • Start Time: 2021-01-27T08:09:38.729+0000

• Duration: 42 min 36 sec

• Commit: fb57be1

Test stats 🧪

Test	Results
Failed	0
Passed	4610
Skipped	127
Total	4737

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[codecov-io]

Codecov Report

Merging #4647 (2e355ff) into master (b456993) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4647      +/-   ##
==========================================
- Coverage   76.10%   76.06%   -0.05%     
==========================================
  Files         161      161              
  Lines        9892     9896       +4     
==========================================
- Hits         7528     7527       -1     
- Misses       2364     2369       +5     

Impacted Files	Coverage Δ
model/metricset.go	95.08% <100.00%> (+0.34%)	⬆️
kibana/connecting_client.go	61.40% <0.00%> (-8.78%)	⬇️

[simitt]

Code looks good and I tested the change - everything works as expected against ES master.

Since _doc_count support was recently introduced, I also tested behavior against 7.9 ES. Writing the docs works fine, but running aggregations doesn't take the values into account. Since this impacts histogram functionality, I assume that Kibana would need to check for the ES version/indices when aggregating?

[dgieselaar]

@simitt Kibana is only expected to work against ES versions of the same minor/major, so it's not expected to support ES 7.9 when running Kibana 7.12.

As transaction histograms are experimental we don't need to worry about backwards compatibility either (AFAIK).

[simitt]

Right! Perfectly fine then.

[axw]

Sorry about the force-push, forgot I was working on a non-draft PR. About to merge anyway.

[axw]

jenkins run the tests please

[axw]

jenkins run the tests please