Add missing godoc strings (golint)

This fixes:
metric_options.go:7:6: exported type CounterOption should have comment or be unexported
metric_options.go:18:1: exported function WithConstLabels should have comment or be unexported
metric_options.go:24:6: exported type HistogramOption should have comment or be unexported
metric_options.go:31:1: exported function WithHistogramConstLabels should have comment or be unexported

Author: knweiss

metric_options.go

@@ -4,7 +4,8 @@ import (
 	prom "github.com/prometheus/client_golang/prometheus"
 )
 
-type CounterOption func(opts *prom.CounterOpts)
+// A CounterOption lets you add options to Counter metrics using With* funcs.
+type CounterOption func(*prom.CounterOpts)
 
 type counterOptions []CounterOption
 
@@ -15,19 +16,24 @@ func (co counterOptions) apply(o prom.CounterOpts) prom.CounterOpts {
 	return o
 }
 
+// WithConstLabels allows you to add ConstLabels to Counter metrics.
 func WithConstLabels(labels prom.Labels) CounterOption {
 	return func(o *prom.CounterOpts) {
 		o.ConstLabels = labels
 	}
 }
 
+// A HistogramOption lets you add options to Histogram metrics using With*
+// funcs.
 type HistogramOption func(*prom.HistogramOpts)
 
 // WithHistogramBuckets allows you to specify custom bucket ranges for histograms if EnableHandlingTimeHistogram is on.
 func WithHistogramBuckets(buckets []float64) HistogramOption {
 	return func(o *prom.HistogramOpts) { o.Buckets = buckets }
 }
 
+// WithHistogramConstLabels allows you to add custom ConstLabels to
+// histograms metrics.
 func WithHistogramConstLabels(labels prom.Labels) HistogramOption {
 	return func(o *prom.HistogramOpts) {
 		o.ConstLabels = labels