Add counter for td-agent warnings
the td-agent log on our vms have many warnings about unprocessed logs.
```
fog/storage/google_json/requests/get_object_url.rb:8:in `get_object_url'\"])"
2020-02-19 11:36:03 +0000 [warn]: #0 pattern not matched: "gitlab-zip-metadata: not a zip"
2020-02-19 11:36:11 +0000 [warn]: #0 pattern not matched: "[fog][DEPRECATION] Fog::Storage::Google => #get_object_url is deprecated, use #get_object_https_url instead ([\"/opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/fog-google-1.9.1/lib/fog/storage/google_json/requests/get_object_url.rb:8:in `get_object_url'\"])"
2020-02-19 11:36:12 +0000 [warn]: #0 pattern not matched: "gitlab-zip-metadata: not a zip"
2020-02-19 11:36:14 +0000 [warn]: #0 pattern not matched: "[fog][DEPRECATION] Fog::Storage::Google => #get_object_url is deprecated, use #get_object_https_url instead ([\"/opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/fog-google-1.9.1/lib/fog/storage/google_json/requests/get_object_url.rb:8:in `get_object_url'\"])"
2020-02-19 11:36:14 +0000 [warn]: #0 pattern not matched: "[fog][DEPRECATION] Fog::Storage::Google => #get_object_url is deprecated, use #get_object_https_url instead ([\"/opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/fog-google-1.9.1/lib/fog/storage/google_json/requests/get_object_url.rb:8:in `get_object_url'\"])"
```
we should start tracking this as a general metric and driving these drops down to zero, either by fixing the application or by updating the fluentd config. The first step will be to add a counter with mtail or see if we have an existing fluentd metric.
cc @igorwwwwwwwwwwwwwwwwwwww @mwasilewski-gitlab
issue