metrics: removes delegate field from prometheus timer metric

We often need the ability to update label values on a timer that's in
flight and originally we would create a delegate, that was a prometheus
Observer when the timer is started. This isn't needed as we can just
create the observer on the fly when we want to stop the timer and
observe the duration, allowing us to set the labels that can be passed
in.

instrumentation/metrics/collectors/timer.go

@@ -1,15 +1,20 @@
 package collectors
 
+// TimerHandle is a type used to map running timers for defered observations.
 type TimerHandle struct{}
 
+// TimerOptions is used to configure a timer with labels and histogram boundaries.
 type TimerOptions struct {
 	Description         string
 	Labels              []string
 	HistogramBoundaries []float64
 }
 
+// Timer is a named metric with labels. It allows concurrent use by returning a handle
+// when the timer is started. This handle is used to lookup a running timer to record the duration.
 type Timer interface {
-	WithLabels([]Label) (Timer, error)
+	// WithLabels returns a new Timer with labels attached.
+	WithLabels(...Label) Timer
 	Start() *TimerHandle
-	ObserveDuration(*TimerHandle)
+	ObserveDuration(*TimerHandle, ...Label)
 }

instrumentation/metrics/internal/noop/timer.go

@@ -4,6 +4,6 @@ import "github.com/puppetlabs/horsehead/v2/instrumentation/metrics/collectors"
 
 type Timer struct{}
 
-func (n Timer) WithLabels([]collectors.Label) (collectors.Timer, error) { return n, nil }
-func (n Timer) Start() *collectors.TimerHandle                          { return &collectors.TimerHandle{} }
-func (n Timer) ObserveDuration(*collectors.TimerHandle)                 {}
+func (n Timer) WithLabels(...collectors.Label) collectors.Timer              { return n }
+func (n Timer) Start() *collectors.TimerHandle                               { return &collectors.TimerHandle{} }
+func (n Timer) ObserveDuration(*collectors.TimerHandle, ...collectors.Label) {}

instrumentation/metrics/internal/prometheus/timer.go

@@ -4,49 +4,48 @@ import (
 	"sync"
 
 	prom "github.com/prometheus/client_golang/prometheus"
-	"github.com/puppetlabs/horsehead/v2/instrumentation/errors"
 	"github.com/puppetlabs/horsehead/v2/instrumentation/metrics/collectors"
 )
 
 type Timer struct {
-	vector   prom.ObserverVec
-	delegate prom.Observer
-	timers   map[*collectors.TimerHandle]*prom.Timer
+	vector prom.ObserverVec
+	timers map[*collectors.TimerHandle]*prom.Timer
+	labels []collectors.Label
 
 	sync.RWMutex
 }
 
-func (t *Timer) WithLabels(labels []collectors.Label) (collectors.Timer, error) {
-	delegate, err := t.vector.GetMetricWith(convertLabels(labels))
-	if err != nil {
-		return nil, errors.NewMetricsUnknownError("prometheus").WithCause(err)
-	}
-
+func (t *Timer) WithLabels(labels ...collectors.Label) collectors.Timer {
 	return &Timer{
-		vector:   t.vector,
-		delegate: delegate,
-		timers:   make(map[*collectors.TimerHandle]*prom.Timer),
-	}, nil
+		vector: t.vector,
+		labels: labels,
+		timers: make(map[*collectors.TimerHandle]*prom.Timer),
+	}
 }
 
 func (t *Timer) Start() *collectors.TimerHandle {
 	t.Lock()
 	defer t.Unlock()
 
 	h := &collectors.TimerHandle{}
+
 	promt := prom.NewTimer(prom.ObserverFunc(func(v float64) {
-		t.delegate.Observe(v)
+		// we can change the label values while a timer is in flight. this allows us to capture
+		// context about what happened inside a callback.
+		t.vector.With(convertLabels(t.labels)).Observe(v)
 	}))
 
 	t.timers[h] = promt
 
 	return h
 }
 
-func (t *Timer) ObserveDuration(h *collectors.TimerHandle) {
+func (t *Timer) ObserveDuration(h *collectors.TimerHandle, labels ...collectors.Label) {
 	t.RLock()
 	defer t.RUnlock()
 
+	t.labels = labels
+
 	if promt, ok := t.timers[h]; ok {
 		promt.ObserveDuration()
 	}

instrumentation/metrics/metrics.go

@@ -91,12 +91,7 @@ func (m *Metrics) MustTimer(name string, labels ...collectors.Label) collectors.
 		return noop.Timer{}
 	}
 
-	t, err = t.WithLabels(labels)
-	if err != nil {
-		m.handleError(err)
-
-		return noop.Timer{}
-	}
+	t = t.WithLabels(labels...)
 
 	return t
 }