Allow using specific Prometheus registerers

[a-random-lemurian]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @a-random-lemurian, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Prometheus middleware by introducing the capability to specify a custom Prometheus Registerer. This change provides greater flexibility for users who need to manage metrics with isolated registries, moving away from the default global Registerer for all metric registrations. It allows for more advanced and controlled Prometheus setups within applications.

Highlights

• Configurable Prometheus Registerer: The Prometheus middleware now accepts an optional prometheus.Registerer in its configuration (Config.Registry), allowing users to provide their own registry instance instead of relying solely on the default global one. If no custom registry is provided, it defaults to prometheus.DefaultRegisterer.
• Dynamic Metric Registration: The internal metric registration logic within the middleware has been updated to use the configured registerer field, ensuring that all metrics are registered with the specified Prometheus registry.
• Improved Test Coverage: A new test case (TestPrometheusMiddlewareCustomRegistry) has been added to specifically validate the functionality of using a custom Prometheus registry, ensuring the new feature works as expected.

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[gemini-code-assist]

Code Review

This pull request adds support for custom Prometheus registerers, which is a great enhancement. The implementation correctly registers metrics with the provided registerer. However, I've identified a critical issue where the /metrics endpoint doesn't serve metrics from this custom registerer, making the feature incomplete. I've provided a detailed fix for this in my review. Additionally, there's a minor suggestion to improve code style for better readability.

[gemini-code-assist]

This new test for a custom registry will fail because the feature is incomplete. The /metrics endpoint is not configured to use the custom registry to serve metrics.

The metrics are correctly registered with the custom reg, but the prometheusHandler() function in middleware.go is hardcoded to use promhttp.Handler(), which serves metrics from the global prometheus.DefaultGatherer. As a result, the check for requests_total inside runRestOfMiddlewareTest will fail for this test case.

To fix this, prometheusHandler should be a method on *Prometheus that uses the configured registerer to serve metrics. The registerer will need to be type-asserted to a prometheus.Gatherer.

Here's a suggested implementation for middleware.go:

1. Change prometheusHandler to be a method on *Prometheus:

   func (p *Prometheus) prometheusHandler() gin.HandlerFunc {
       gatherer, ok := p.registerer.(prometheus.Gatherer)
       if !ok {
           log.Warn("provided prometheus.Registerer does not implement prometheus.Gatherer, using default gatherer for /metrics endpoint")
           h := promhttp.Handler()
           return func(c *gin.Context) {
               h.ServeHTTP(c.Writer, c.Request)
           }
       }
   
       h := promhttp.HandlerFor(gatherer, promhttp.HandlerOpts{})
       return func(c *gin.Context) {
           h.ServeHTTP(c.Writer, c.Request)
       }
   }

2. Update SetMetricsPath and SetMetricsPathWithAuth to call p.prometheusHandler() instead of prometheusHandler().

With these changes in middleware.go, this test should pass.

[gemini-code-assist]

This logic for setting the default registerer can be made more concise and idiomatic by first assigning the configured registry and then checking for nil to apply the default.

Suggested change

	p.registerer = prometheus.DefaultRegisterer
	if cfg.Registry != nil {
	p.registerer = cfg.Registry
	}
	p.registerer = cfg.Registry
	if p.registerer == nil {
	p.registerer = prometheus.DefaultRegisterer
	}