Heartbeat: fix browser metrics and trace mappings

[vigneshshanmugam]

• fixes [Heartbeat] Move browser fields to root #30095
• Move the browser.experience and browser.relative_trace to the root HB mapping fields.

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• Start Time: 2022-02-15T16:15:55.079+0000

• Duration: 79 min 13 sec

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Total	2659

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This pull request is now in conflicts. Could you fix it? 🙏
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git fetch upstream
git checkout -b update-synth-mapping upstream/update-synth-mapping
git merge upstream/main
git push upstream update-synth-mapping

[lucasfcosta]

I have E2E tested this and compared these changes to the mappings for the synthetics integration. It LGTM.

My E2E test consisted of:

1. Creating a Heartbeat config file with browser monitors
2. Checking out this branch and running mage build
3. Running the built Heartbeat against my local ES
4. Checking the documents sent on the Kibana Dev Console

   GET /.ds-heartbeat-8.2.0-2022.02.15-000001/_search
   {
     "size": 50, 
     "query": {
       "match": {
         "event.type": "step/metrics"
       }
     }
   }
   

5. Checking the mappings themselves in the index management screen
   
6. Making sure the monitors themselves work fine (including the waterfall chart)
   

I'd appreciate it if @dominiqueclarke could just confirm these look OK on her side too, but no need to E2E test I'd say.