Promtail: Ignore dropped entries in subsequent metric-stages in pipelines.

[dtennander]

What this PR does / why we need it:
This PR makes the metric stage in Promtail ignore dropped entries.

This allows users to define a pipeline like this:

pipeline_stages:
      - match:
          selector: '{app="promtail"} |= "thing we don't want to log"'
          action: drop
      - metrics:
          log_lines_total:
            type: Counter
            description: "total number of logged lines"
            config:
              match_all: true
              action: inc

In general this also fixes the issue were the above config today would generate an error message like this:

missmatch metrics: gathering metrics failed: "__drop__" is not a valid label name for metric "promtail_custom_log_lines_total"

Which issue(s) this PR fixes:
Doesn't fix a GH-Issue 😢

Special notes for your reviewer:
I also have a branch feature/metrics-on-dropped that solves this issue by removing labels with the prefix __ instead. But I do believe that this PR solves the problem in a more predictable way.

Checklist

• Documentation added
• Tests updated

[claassistantio]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #1743 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1743      +/-   ##
==========================================
+ Coverage   64.22%   64.25%   +0.02%     
==========================================
  Files         121      121              
  Lines        9083     9085       +2     
==========================================
+ Hits         5834     5838       +4     
+ Misses       2846     2845       -1     
+ Partials      403      402       -1

Impacted Files	Coverage Δ
pkg/logentry/stages/metrics.go	60.32% <100%> (+0.43%)	⬆️
pkg/ingester/transfer.go	66.42% <0%> (+1.42%)	⬆️

[cyriltovena]

LGTM Awesome thank you for this.