feat(instrumentation): allow passing in any prometheus.Registerer (#369)

The prometheus client_go library primarily uses the prometheus.Registerer
interface instead of the prometheus.Registry struct for registering
metrics.

Especially the global default registry is only exposed as the interfaces
`prometheus.Registerer` and `prometheus.Gatherer`[0]. Users that want to
use the default registry currently need to make a type assertion to get
the struct from the exported interface:

    hcloud.WithInstrumentation(prometheus.DefaultGatherer.(*prometheus.Registry))

This could fail at any point if the library changes the underlying
implmentation to a different struct.

controller-runtime is another example of an SDK that only exports the
registry as the interface. I had to implement the above type assertation
to get hcloud-go metrics in cluster-api-provider-hetzner[1].

This will not break any user code, because the struct we currently
accept implements the interface we will accept from now on.

[0]: https://github.com/prometheus/client_golang/blob/046e3203b61d1b5000b162e210a91975f098d742/prometheus/registry.go#L54-L58
[1]: https://github.com/syself/cluster-api-provider-hetzner/blob/b421af694d62a6b66ecbb7a6db2ea1f78a620179/pkg/services/hcloud/client/client.go#L88-L92

hcloud/client.go

@@ -65,7 +65,7 @@ type Client struct {
 	applicationVersion      string
 	userAgent               string
 	debugWriter             io.Writer
-	instrumentationRegistry *prometheus.Registry
+	instrumentationRegistry prometheus.Registerer
 
 	Action           ActionClient
 	Certificate      CertificateClient
@@ -163,7 +163,7 @@ func WithHTTPClient(httpClient *http.Client) ClientOption {
 }
 
 // WithInstrumentation configures a Client to collect metrics about the performed HTTP requests.
-func WithInstrumentation(registry *prometheus.Registry) ClientOption {
+func WithInstrumentation(registry prometheus.Registerer) ClientOption {
 	return func(client *Client) {
 		client.instrumentationRegistry = registry
 	}

hcloud/internal/instrumentation/metrics.go

@@ -13,11 +13,11 @@ import (
 
 type Instrumenter struct {
 	subsystemIdentifier     string // will be used as part of the metric name (hcloud_<identifier>_requests_total)
-	instrumentationRegistry *prometheus.Registry
+	instrumentationRegistry prometheus.Registerer
 }
 
 // New creates a new Instrumenter. The subsystemIdentifier will be used as part of the metric names (e.g. hcloud_<identifier>_requests_total).
-func New(subsystemIdentifier string, instrumentationRegistry *prometheus.Registry) *Instrumenter {
+func New(subsystemIdentifier string, instrumentationRegistry prometheus.Registerer) *Instrumenter {
 	return &Instrumenter{subsystemIdentifier: subsystemIdentifier, instrumentationRegistry: instrumentationRegistry}
 }
 
@@ -84,7 +84,7 @@ func (i *Instrumenter) instrumentRoundTripperEndpoint(counter *prometheus.Counte
 // registerOrReuse will try to register the passed Collector, but in case a conflicting collector was already registered,
 // it will instead return that collector. Make sure to always use the collector return by this method.
 // Similar to [Registry.MustRegister] it will panic if any other error occurs.
-func registerOrReuse[C prometheus.Collector](registry *prometheus.Registry, collector C) C {
+func registerOrReuse[C prometheus.Collector](registry prometheus.Registerer, collector C) C {
 	err := registry.Register(collector)
 	if err != nil {
 		// If we get a AlreadyRegisteredError we can return the existing collector

hcloud/metadata/client.go

@@ -22,7 +22,7 @@ type Client struct {
 	timeout  time.Duration
 
 	httpClient              *http.Client
-	instrumentationRegistry *prometheus.Registry
+	instrumentationRegistry prometheus.Registerer
 }
 
 // A ClientOption is used to configure a [Client].
@@ -43,7 +43,7 @@ func WithHTTPClient(httpClient *http.Client) ClientOption {
 }
 
 // WithInstrumentation configures a [Client] to collect metrics about the performed HTTP requests.
-func WithInstrumentation(registry *prometheus.Registry) ClientOption {
+func WithInstrumentation(registry prometheus.Registerer) ClientOption {
 	return func(client *Client) {
 		client.instrumentationRegistry = registry
 	}