Prometheus collector in metricbeat silently fails if metric name is incorrect

[jsoriano]

Prometheus collector silently fails if the prometheus response contains invalid metric
names, this can be reproduced for example if the metric name contain dots.
This error should be reported, or at least logged on the debug level.

For confirmed bugs, please report:

• Version: 6.X, 7.X
• Steps to Reproduce: Configure the prometheus collector to an endpoint that exposes
  prometheus metrics with incorrect names. Seen in java applications with Micrometer
  exposing metrics with dots in their names.

[kaiyan-sheng]

#13353 fixed this.