Add tags field to internal_metrics data stream

[axw]

Motivation/summary

Documents added to metrics-apm.internal-<namespace> may have client.ip, and may require geoIP enrichment. If the geoip ingest processor cannot download the database, it will add a tag to the document. As we do not have the tags field defined, and we use strict mapping for this data stream, this leads to an indexing error.

Checklist

- [ ] Update CHANGELOG.asciidoc

• Update package changelog.yml (only if changes to apmpackage have been made)
  - [ ] Documentation has been updated

How to test these changes

1. Modify docker-compose.yml's Elasticsearch service, adding ingest.geoip.downloader.enabled=false to its environment and removing the ingest-geoip volume.
2. Send some metrics to the RUM events endpoint with a public IP set. e.g. curl -H "X-Real-Ip: 8.8.8.8" -X POST -H Content-Type:application/x-ndjson http://localhost:8200/intake/v2/rum/events --data-binary @testdata/intake-v2/metricsets.ndjson

With the fix, metric documents should be indexed with a tags field.

Related issues

Closes #7465

[apmmachine]

💔 Tests Failed

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Build stats

• Start Time: 2022-06-03T02:53:03.948+0000

• Duration: 29 min 7 sec

Test stats 🧪

Test	Results
Failed	1
Passed	4008
Skipped	13
Total	4022

Test errors

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Build and Test / System and Environment Tests / TestAggregate – github.com/elastic/apm-server/systemtest/benchtest/expvar

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 Failed 

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 === RUN   TestAggregate
    metrics_test.go:64: 
        	Error Trace:	metrics_test.go:64
        	Error:      	"48" is not greater than or equal to "72"
        	Test:       	TestAggregate
--- FAIL: TestAggregate (0.06s)
 

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (42/42)	💚
Files	91.96% (183/199)	👍
Classes	93.421% (426/456)	👍
Methods	89.265% (1081/1211)	👍 0.083
Lines	76.818% (13162/17134)	👍 0.035
Conditionals	100.0% (0/0)	💚