feat(metricsserver): add Gatherer method

Add explicit Gatherer() method to improve API discoverability
and prevent users from accidentally using prometheus.DefaultGatherer
instead of the custom registry.

Changes:
- Add Gatherer() method returning prometheus.Gatherer interface
- Add NewWithDefaultGatherer() constructor for opt-in default usage
- Update package docs with usage examples
- Add tests for both gatherer modes

metricsserver/metrics.go

@@ -3,6 +3,24 @@
 // This package implements a dedicated metrics server that exposes application metrics
 // via HTTP. It uses a custom Prometheus registry to avoid conflicts with other metric
 // collectors and provides graceful shutdown capabilities.
+//
+// # Usage
+//
+// Create a new metrics server and register your metrics with its Registry():
+//
+//	m := metricsserver.New()
+//	myCounter := prometheus.NewCounter(...)
+//	m.Registry().MustRegister(myCounter)
+//
+// When you need a Gatherer (for example, to pass to other libraries), use the Gatherer() method
+// instead of prometheus.DefaultGatherer to ensure your custom metrics are collected:
+//
+//	gatherer := m.Gatherer()  // Returns the custom registry as a Gatherer
+//
+// To use the default Prometheus gatherer alongside your custom registry:
+//
+//	m := metricsserver.NewWithDefaultGatherer()
+//	m.Gatherer()  // Returns prometheus.DefaultGatherer
 package metricsserver
 
 import (
@@ -21,15 +39,32 @@ import (
 // Metrics provides a custom Prometheus registry and HTTP handlers for metrics exposure.
 // It isolates application metrics from the default global registry.
 type Metrics struct {
-	r *prometheus.Registry
+	r                  *prometheus.Registry
+	useDefaultGatherer bool
 }
 
 // New creates a new Metrics instance with a custom Prometheus registry.
+// Use this when you want isolated metrics that don't interfere with the default registry.
 func New() *Metrics {
 	r := prometheus.NewRegistry()
 
 	m := &Metrics{
-		r: r,
+		r:                  r,
+		useDefaultGatherer: false,
+	}
+
+	return m
+}
+
+// NewWithDefaultGatherer creates a new Metrics instance that uses the default Prometheus gatherer.
+// This is useful when you want to expose metrics from the default registry alongside your custom metrics.
+// The custom registry will still be available via Registry() but Gatherer() will return prometheus.DefaultGatherer.
+func NewWithDefaultGatherer() *Metrics {
+	r := prometheus.NewRegistry()
+
+	m := &Metrics{
+		r:                  r,
+		useDefaultGatherer: true,
 	}
 
 	return m
@@ -41,6 +76,23 @@ func (m *Metrics) Registry() *prometheus.Registry {
 	return m.r
 }
 
+// Gatherer returns the Prometheus gatherer to use for collecting metrics.
+// This returns the custom registry's Gatherer by default, ensuring your registered
+// metrics are collected. If the instance was created with NewWithDefaultGatherer(),
+// this returns prometheus.DefaultGatherer instead.
+//
+// Use this method when you need a prometheus.Gatherer interface, for example when
+// configuring other libraries that need to collect metrics.
+//
+// IMPORTANT: Do not use prometheus.DefaultGatherer directly if you want to collect
+// metrics registered with this instance's Registry(). Always use this Gatherer() method.
+func (m *Metrics) Gatherer() prometheus.Gatherer {
+	if m.useDefaultGatherer {
+		return prometheus.DefaultGatherer
+	}
+	return m.r
+}
+
 // Handler returns an HTTP handler for the /metrics endpoint with OpenMetrics support.
 func (m *Metrics) Handler() http.Handler {
 	log := logger.NewStdLog("prom http", false, nil)