feat: add metrics system, move webhook feature to dedicated module (#372)

pkg/gotenberg/metrics.go

@@ -0,0 +1,26 @@
+package gotenberg
+
+// Metric represents a unitary metric.
+type Metric struct {
+	// Name is the unique identifier.
+	// Required.
+	Name string
+
+	// Description describes the metric.
+	// Optional.
+	Description string
+
+	// Read returns the current value.
+	// Required.
+	Read func() float64
+}
+
+// MetricsProvider is a module interface which provides a list of Metric.
+//
+//	func (m *YourModule) Provision(ctx *gotenberg.Context) error {
+//		provider, _ := ctx.Module(new(gotenberg.MetricsProvider))
+//		metrics, _  := provider.(gotenberg.MetricsProvider).Metrics()
+//	}
+type MetricsProvider interface {
+	Metrics() ([]Metric, error)
+}