Add fetching counter metric type

metrics/metric_counter_fetching.go

@@ -0,0 +1,57 @@
+package metrics
+
+import (
+	"fmt"
+	"io"
+
+	vm "github.com/VictoriaMetrics/metrics"
+)
+
+// FetchingCounter is a counter metric that fetches the values via a function call.
+type FetchingCounter struct {
+	*metricBase
+	counter  *vm.Counter
+	fetchCnt func() uint64
+}
+
+// NewFetchingCounter registers a new fetching counter metric.
+func NewFetchingCounter(id string, labels map[string]string, fn func() uint64, opts *Options) (*FetchingCounter, error) {
+	// Check if a fetch function is provided.
+	if fn == nil {
+		return nil, fmt.Errorf("%w: no fetch function provided", ErrInvalidOptions)
+	}
+
+	// Ensure that there are options.
+	if opts == nil {
+		opts = &Options{}
+	}
+
+	// Make base.
+	base, err := newMetricBase(id, labels, *opts)
+	if err != nil {
+		return nil, err
+	}
+
+	// Create metric struct.
+	m := &FetchingCounter{
+		metricBase: base,
+		fetchCnt:   fn,
+	}
+
+	// Create metric in set
+	m.counter = m.set.NewCounter(m.LabeledID())
+
+	// Register metric.
+	err = register(m)
+	if err != nil {
+		return nil, err
+	}
+
+	return m, nil
+}
+
+// WritePrometheus writes the metric in the prometheus format to the given writer.
+func (fc *FetchingCounter) WritePrometheus(w io.Writer) {
+	fc.counter.Set(fc.fetchCnt())
+	fc.metricBase.set.WritePrometheus(w)
+}