Cherry-pick #17104 to 7.x: [Metricbeat] Add 'query' metricset for prometheus module

[ChrsMark]

Cherry-pick of PR #17104 to 7.x branch. Original message:

What does this PR do?

Adds 'query' metricset for prometheus module. It continues the PR #15177, and aims to the cleanups/changes required.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

1. Start a Prometheus container: docker run -p 9090:9090 prom/prometheus
2. Enable Prometheus module of Metricbeat and test with the following cases:
   a.

- module: prometheus
  period: 10s
  hosts: ["localhost:9090"]
  metricsets: ["query"]
  queries:
  - query_name: "string"
    path: "/api/v1/query"
    query_params:
      query: "some_value"

b.

- module: prometheus
  period: 10s
  hosts: ["localhost:9090"]
  metricsets: ["query"]
  queries:
  - query_name: "scalar"
    path: "/api/v1/query"
    query_params:
      query: "100"

c.

- module: prometheus
  period: 10s
  hosts: ["localhost:9090"]
  metricsets: ["query"]
  queries:
  - query_name: "instant_vector"
    path: "/api/v1/query"
    query_params:
      query: "sum(rate(prometheus_http_requests_total[1m]))"

d.
Change time range accordingly. Also some time is needed so the the metrics to have been populated in Prometheus for the respective range

- module: prometheus
  period: 10s
  hosts: ["localhost:9090"]
  metricsets: ["query"]
  queries:
  - query_name: "range_vector"
    path: "/api/v1/query_range"
    query_params:
      query: "up"
      start: "2020-03-20T00:00:00.000Z"
      end:  "2020-03-20T00:00:10.000Z"
      step: 1m

Related issues

• Superseds [Metricbeat] Add 'query' metricset for prometheus module #15177

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)