Monitoring data is not indexed in ES

[bryce-b]

APM Server version (apm-server version): v7.16.0-SNAPSHOT

Description of the problem including expected versus actual behavior:

Data captured in .monitoring-beats* indices under beats_stats.metrics.apm-server.sampling.* is not indexed and can not be queried in the Console or charts.

Steps to reproduce:

1. enable tail-based sampling in the apm-server
   e.g.:

apm-server:
  data_streams:
    enabled: true
  sampling:
    keep_unsampled: false
    tail:
      enabled: true
      ttl: 30s
      interval: 1s
      policies: 
        - sample_rate: 0.1 

2. enable stack monitoring
3. try to query beats_stats.metrics.apm-server.sampling.*
   e.g.:

GET .monitoring-beats-7-2021.11.11/_search
{
  "query": {
    "bool": {
      "filter": [
        { 
          "exists": {
            "field": "beats_stats.metrics.apm-server.sampling"
          }
        }
      ]
    }
  }
}

Provide logs (if relevant):

[axw]

We'll need to update the beats monitoring template in Elasticsearch.

[axw]

The PR for this is in the Elasticsearch repo. I'll label this issue for the test plan now so we don't forget.