Add a counter for empty syslog messages

[carlpett]

What this PR does / why we need it:
This PR adds a counter for empty syslog messages.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

[codecov-commenter]

Codecov Report

Merging #2565 into master will increase coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2565      +/-   ##
==========================================
+ Coverage   62.89%   62.96%   +0.06%     
==========================================
  Files         169      169              
  Lines       14911    14912       +1     
==========================================
+ Hits         9378     9389      +11     
+ Misses       4788     4774      -14     
- Partials      745      749       +4     

Impacted Files	Coverage Δ
pkg/promtail/targets/syslog/syslogtarget.go	74.34% <0.00%> (-0.50%)	⬇️
pkg/promtail/targets/file/tailer.go	70.83% <0.00%> (-4.17%)	⬇️
pkg/promtail/positions/positions.go	59.64% <0.00%> (+13.15%)	⬆️

[patriklindstrom]

Are there any test that can cover this new code? Otherwise I think this i great. Would help us.

[slim-bean]

Thanks @carlpett! It's a bit tricky to write tests around metrics like this and the code looks good to me!

[slim-bean]

LGTM