add ability to use prometheus compat histogram

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

victoriametrics_test.go

@@ -1,6 +1,7 @@
 package victoriametrics
 
 import (
+	"bytes"
 	"testing"
 )
 
@@ -16,3 +17,13 @@ func TestBuildName(t *testing.T) {
 	cnt := m.Counter("counter", "key", "val")
 	cnt.Inc()
 }
+
+func TestPrometheusCompat(t *testing.T) {
+	m := NewMeter(PrometheusCompat(true))
+	m.Histogram("foo", "key", "val").Update(15)
+
+	buf := bytes.NewBuffer(nil)
+
+	_ = m.Write(buf)
+	t.Logf("\n%s", buf.Bytes())
+}