Make GroupValue() more resilient

[chr4]

When sending a topic that doesn't match the configured regex, mqtt2prometheus currently just crashes.

This can be reproduced by using this config and then publishing a message to shellies/test:

mqtt:
  server: tcp://mosquitto:1883
  topic_path: shellies/#
  device_id_regex: "shellies/(?P<deviceid>.*)/sensor"
  metric_per_topic_config:
    metric_name_regex: "shellies/(?P<deviceid>.*)/sensor/(?P<metricname>.*)"

This patch ignores topic not parseable by the regex instead and outputs the following error instead:

error        cmd/mqtt2prometheus.go:119        Error while processing message
{"error": "could not store metrics '' on topic shellies/test: failed to extract metric values from topic: failed to find valid metric in topic path"}

[chr4]

Thanks for merging so quickly!

Is there a release planned with this fix?

[hikhvar]

In short yes. I'm currently writing some regression test for this and other functions.

[hikhvar]

I just tagged the current master with v0.1.6-RC3. After the github action run the new release should be ready.
For a v0.1.6 release I want to switch to a rootless Docker image.

[hikhvar]

Here it is: https://github.com/hikhvar/mqtt2prometheus/releases/tag/v0.1.6-RC3