Pull `With` and `WithLabelValues` up into exported types

The "panic in case of error" code was so far in metricVec. This pulls
it up into the exported types like CounterVec. This is code
replication, but it avoids an explicit type conversion. Mostly,
however, this is preparation to make the wrapped metricVec an
interface (required for curried vec's).

prometheus/counter.go

@@ -152,14 +152,22 @@ func (m *CounterVec) GetMetricWith(labels Labels) (Counter, error) {
 // error, WithLabelValues allows shortcuts like
 //     myVec.WithLabelValues("404", "GET").Add(42)
 func (m *CounterVec) WithLabelValues(lvs ...string) Counter {
-	return m.metricVec.withLabelValues(lvs...).(Counter)
+	c, err := m.GetMetricWithLabelValues(lvs...)
+	if err != nil {
+		panic(err)
+	}
+	return c
 }
 
 // With works as GetMetricWith, but panics where GetMetricWithLabels would have
 // returned an error. By not returning an error, With allows shortcuts like
 //     myVec.With(Labels{"code": "404", "method": "GET"}).Add(42)
 func (m *CounterVec) With(labels Labels) Counter {
-	return m.metricVec.with(labels).(Counter)
+	c, err := m.GetMetricWith(labels)
+	if err != nil {
+		panic(err)
+	}
+	return c
 }
 
 // CounterFunc is a Counter whose value is determined at collect time by calling a

prometheus/gauge.go

@@ -138,14 +138,22 @@ func (m *GaugeVec) GetMetricWith(labels Labels) (Gauge, error) {
 // error, WithLabelValues allows shortcuts like
 //     myVec.WithLabelValues("404", "GET").Add(42)
 func (m *GaugeVec) WithLabelValues(lvs ...string) Gauge {
-	return m.metricVec.withLabelValues(lvs...).(Gauge)
+	g, err := m.GetMetricWithLabelValues(lvs...)
+	if err != nil {
+		panic(err)
+	}
+	return g
 }
 
 // With works as GetMetricWith, but panics where GetMetricWithLabels would have
 // returned an error. By not returning an error, With allows shortcuts like
 //     myVec.With(Labels{"code": "404", "method": "GET"}).Add(42)
 func (m *GaugeVec) With(labels Labels) Gauge {
-	return m.metricVec.with(labels).(Gauge)
+	g, err := m.GetMetricWith(labels)
+	if err != nil {
+		panic(err)
+	}
+	return g
 }
 
 // GaugeFunc is a Gauge whose value is determined at collect time by calling a

prometheus/histogram.go

@@ -363,14 +363,22 @@ func (m *HistogramVec) GetMetricWith(labels Labels) (Observer, error) {
 // error, WithLabelValues allows shortcuts like
 //     myVec.WithLabelValues("404", "GET").Observe(42.21)
 func (m *HistogramVec) WithLabelValues(lvs ...string) Observer {
-	return m.metricVec.withLabelValues(lvs...).(Observer)
+	h, err := m.GetMetricWithLabelValues(lvs...)
+	if err != nil {
+		panic(err)
+	}
+	return h
 }
 
 // With works as GetMetricWith, but panics where GetMetricWithLabels would have
 // returned an error. By not returning an error, With allows shortcuts like
 //     myVec.With(Labels{"code": "404", "method": "GET"}).Observe(42.21)
 func (m *HistogramVec) With(labels Labels) Observer {
-	return m.metricVec.with(labels).(Observer)
+	h, err := m.GetMetricWith(labels)
+	if err != nil {
+		panic(err)
+	}
+	return h
 }
 
 type constHistogram struct {

prometheus/summary.go

@@ -480,14 +480,22 @@ func (m *SummaryVec) GetMetricWith(labels Labels) (Observer, error) {
 // error, WithLabelValues allows shortcuts like
 //     myVec.WithLabelValues("404", "GET").Observe(42.21)
 func (m *SummaryVec) WithLabelValues(lvs ...string) Observer {
-	return m.metricVec.withLabelValues(lvs...).(Observer)
+	s, err := m.GetMetricWithLabelValues(lvs...)
+	if err != nil {
+		panic(err)
+	}
+	return s
 }
 
 // With works as GetMetricWith, but panics where GetMetricWithLabels would have
 // returned an error. By not returning an error, With allows shortcuts like
 //     myVec.With(Labels{"code": "404", "method": "GET"}).Observe(42.21)
 func (m *SummaryVec) With(labels Labels) Observer {
-	return m.metricVec.with(labels).(Observer)
+	s, err := m.GetMetricWith(labels)
+	if err != nil {
+		panic(err)
+	}
+	return s
 }
 
 type constSummary struct {

prometheus/vec.go

@@ -89,22 +89,6 @@ func (m *metricVec) getMetricWith(labels Labels) (Metric, error) {
 	return m.getOrCreateMetricWithLabels(h, labels), nil
 }
 
-func (m *metricVec) withLabelValues(lvs ...string) Metric {
-	metric, err := m.getMetricWithLabelValues(lvs...)
-	if err != nil {
-		panic(err)
-	}
-	return metric
-}
-
-func (m *metricVec) with(labels Labels) Metric {
-	metric, err := m.getMetricWith(labels)
-	if err != nil {
-		panic(err)
-	}
-	return metric
-}
-
 // DeleteLabelValues removes the metric where the variable labels are the same
 // as those passed in as labels (same order as the VariableLabels in Desc). It
 // returns true if a metric was deleted.