Cherry-pick #16609 to 7.x: Prometheus remote write endpoint

[ChrsMark]

Cherry-pick of PR #16609 to 7.x branch. Original message:

What does this PR do?

This PR implements Prometheus remote write as a new metricset.

Why is it important?

How to test this PR locally

Start metricbeat

1. Enable metricbeat Prometheus module
2. config:

- module: prometheus
  period: 10s
  metricsets: ["remote_write"]

3. start metricbeat and expect to see:

2020-02-27T17:49:14.724+0200	DEBUG	[module]	module/wrapper.go:189	Starting metricSetWrapper[module=prometheus, name=remote_write, host=]
2020-02-27T17:49:14.724+0200	INFO	remote_write/remote_write.go:92	Starting HTTP server on localhost:9201

Sending metrics from Prometheus

Use this forked project: https://github.com/ChrsMark/dockprom.

1. Download the project.
2. Add to `prometheus/prometheus.yml:

remote_write:
  - url: "http://host.docker.internal:9201/write"

3. docker-compose up

With that metrics should be collected by metricbeat and reported.

Related issues

• Closes [Metricbeat] Prometheus Remote Endpoint #14983

Notes about changes

This PR changes HttpServer so as to accept handlerFunc from metricset on server initialisation. This makes it reusable from different metricsets and not only server metricset of http module. In order to confirm that server metricset of http module is not broken I run INTEGRATION_TESTS=1 BEAT_STRICT_PERMS=false nosetests ./module/http/test_http.py successfully which shows that functionality is preserved.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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