[RUM V3] Correctly process tags in RUMV3 metricsets.

[jalvz]

Also, add missing fields for approval tests.

Motivation/summary

Adds to the rum v3 sample document any missing fields. Doing this, I found out that tags in metricsets were not abbreviated.
It is not too bad because RUM doesn't use them yet, but it is fixed here.

Checklist

• I have signed the Contributor License Agreement.
• I have updated CHANGELOG.asciidoc

I have considered changes for:

• documentation
• logging (add log lines, choose appropriate log selector, etc.)
• metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)
• automated tests (add tests for the code changes, all unit tests pass locally)
• telemetry
• Elasticsearch Service (https://cloud.elastic.co)
• Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

Related issues

closes #3476

[codecov-commenter]

Codecov Report

Merging #3816 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3816      +/-   ##
==========================================
+ Coverage   79.81%   79.89%   +0.08%     
==========================================
  Files         136      136              
  Lines        6158     6158              
==========================================
+ Hits         4915     4920       +5     
+ Misses       1243     1238       -5     

Impacted Files	Coverage Δ
model/modeldecoder/metricset.go	83.33% <100.00%> (ø)
kibana/connecting_client.go	72.58% <0.00%> (+8.06%)	⬆️

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #3816 opened]

• Start Time: 2020-05-25T12:32:05.275+0000

• Duration: 45 min 54 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3227
Skipped	145
Total	3372

Steps errors

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• Name: Test Sync

  • Description: ./script/jenkins/sync.sh

  • Duration: 3 min 53 sec

  • Start Time: 2020-05-25T12:42:02.144+0000

  • log