feat: add support for custom labels in Prometheus metrics

[appleboy]

• Add toPtr function to convert a bool to a bool pointer
• Add contains function to check if a key exists in a slice
• Add GetMetricLabels method to Config struct to return unique metric labels from enabled endpoints
• Change file permission notation from 0644 to 0o644 in config_test.go
• Add Labels field to Endpoint struct for key-value pairs
• Initialize Prometheus metrics with dynamic labels from configuration
• Modify PublishMetricsForEndpoint to include dynamic labels
• Add test for GetMetricLabels method in config_test.go
• Update watchdog to pass labels to monitoring and execution functions

Summary

The changes for config.yaml

endpoints:
  - name: front-end
    group: core
    url: "https://twin.sh/health"
    interval: 5m
    conditions:
      - "[STATUS] == 200"
      - "[BODY].status == UP"
      - "[RESPONSE_TIME] < 150"
+   labels:
+     environment: staging

This pull request introduces several enhancements to the Gatus project, including the addition of a Labels field to the Config and Endpoint structs to support key-value pairs for metrics. It also updates the Prometheus metrics initialization to include dynamic labels from the configuration, modifies the PublishMetricsForEndpoint function to handle and include these labels, and adds corresponding tests and updates to the codebase to ensure the functionality works as intended.

New metrics preview

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[TwiN]

Hey @appleboy, thanks again for your contributions to Gatus!

What use does the global labels have vs, for instance, just pushing the labels defined in the endpoint config?

In other words, if the environment label is defined in the endpoint, well, its only use is for metrics, so if the user defined it in the endpoint config, it's because they explicitly want it and therefore it should be published with the metrics, which would then make the global/top-level labels configuration unnecessary, no?

Let me know if the comment above doesn't make sense, I'm running on very little sleep today 😅

[appleboy]

@TwiN

Thank you for your response. I was also thinking about the issue you mentioned yesterday. I will further modify it so that if the endpoint is configured, it will be automatically added to the metrics.

[appleboy]

@TwiN Updated. Please review it again. I also add config file unit testing.

[appleboy]

@TwiN Any feedbacks?

[appleboy]

@TwiN Done. See the changes 85c5e73