Cherry-pick #15712 to 7.6: [Metricbeat]Log prometheus errors instead of parsing families

[jsoriano]

Cherry-pick of PR #15712 to 7.6 branch. Original message:

Solves #15709

Some users have been reporting errors fetching prometheus endpoints that include logs stating format errors decoding families.

Most probably metricbeat's prometheus helper has been trying to parse errors received as metric families. This PR adds the HTTP status code to the error sent upstream from the helper and also allows for writing the error body to log when debug is enabled.

[jsoriano]

jenkins, test this again please

[jsoriano]

jenkins, test this again please

This is failing in jenkins, could be related as state_deployment uses prometheus, but this doesn't fail locally or in travis 🤔

16:21:22 command [go test -race -cover -coverprofile /tmp/gotestcover-618583823 github.com/elastic/beats/metricbeat/mb/testing/data]: exit status 1
16:21:22 --- FAIL: TestAll (96.86s)
16:21:22     --- FAIL: TestAll/kubernetes.state_deployment (15.38s)
16:21:22         --- FAIL: TestAll/kubernetes.state_deployment/ksm-v1.8.0.plain (13.32s)
16:21:22             testdata.go:272: Event was not expected: {"error":{"message":"decoding of metric family failed: net/http: request canceled (Client.Timeout exceeded while reading body)"},"event":{"dataset":"kubernetes.state_deployment","duration":115000,"module":"kubernetes"},"metricset":{"name":"state_deployment","period":10000},"service":{"address":"127.0.0.1:55555","type":"kubernetes"}}