[7.x] Add support for dynamic histogram metrics (backport #5239)

[mergify]

This is an automatic backport of pull request #5239 done by Mergify.
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On branch mergify/bp/7.x/pr-5239
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  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	new file:   apmpackage/apm/0.2.0/data_stream/app_metrics/elasticsearch/ingest_pipeline/apm_metrics_dynamic_template.json
	modified:   apmpackage/apm/0.2.0/data_stream/app_metrics/elasticsearch/ingest_pipeline/default.json
	modified:   apmpackage/apm/0.2.0/data_stream/app_metrics/fields/fields.yml
	new file:   apmpackage/apm/0.2.0/data_stream/error_logs/elasticsearch/ingest_pipeline/apm_metrics_dynamic_template.json
	modified:   apmpackage/apm/0.2.0/data_stream/error_logs/elasticsearch/ingest_pipeline/default.json
	new file:   apmpackage/apm/0.2.0/data_stream/internal_metrics/elasticsearch/ingest_pipeline/apm_metrics_dynamic_template.json
	modified:   apmpackage/apm/0.2.0/data_stream/internal_metrics/elasticsearch/ingest_pipeline/default.json
	modified:   apmpackage/apm/0.2.0/data_stream/internal_metrics/fields/fields.yml
	new file:   apmpackage/apm/0.2.0/data_stream/profile_metrics/elasticsearch/ingest_pipeline/apm_metrics_dynamic_template.json
	modified:   apmpackage/apm/0.2.0/data_stream/profile_metrics/elasticsearch/ingest_pipeline/default.json
	new file:   apmpackage/apm/0.2.0/data_stream/traces/elasticsearch/ingest_pipeline/apm_metrics_dynamic_template.json
	modified:   apmpackage/apm/0.2.0/data_stream/traces/elasticsearch/ingest_pipeline/default.json
	modified:   apmpackage/apm/0.2.0/docs/README.md
	modified:   docs/fields.asciidoc
	modified:   ingest/pipeline/definition.json
	modified:   ingest/pipeline/definition.yml
	modified:   model/metricset.go
	modified:   model/metricset/_meta/fields.yml
	modified:   model/metricset_test.go
	modified:   processor/otel/metrics.go
	modified:   processor/otel/metrics_test.go
	modified:   systemtest/approvals/TestOTLPGRPCMetrics.approved.json
	modified:   systemtest/otlp_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   changelogs/head.asciidoc
	both modified:   include/fields.go

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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Build stats

• Build Cause: Pull request #5352 updated

• Start Time: 2021-05-27T02:51:09.487+0000

• Duration: 51 min 5 sec

• Commit: 36df153

Test stats 🧪

Test	Results
Failed	0
Passed	4787
Skipped	121
Total	4908

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