Switch from counter to a gauge for partitions held

Counters cannot be decremented in Prometheus:

```
panic: counter cannot decrease in value

goroutine 895 [running]:
github.com/jaegertracing/jaeger/vendor/github.com/prometheus/client_golang/prometheus.(*counter).Add(0xc000790600, 0xbff0000000000000)
    /home/travis/gopath/src/github.com/jaegertracing/jaeger/vendor/github.com/prometheus/client_golang/prometheus/counter.go:71 +0xa3
github.com/jaegertracing/jaeger/vendor/github.com/uber/jaeger-lib/metrics/prometheus.(*counter).Inc(0xc0006b42a0, 0xffffffffffffffff)
    /home/travis/gopath/src/github.com/jaegertracing/jaeger/vendor/github.com/uber/jaeger-lib/metrics/prometheus/factory.go:183 +0x46
github.com/jaegertracing/jaeger/cmd/ingester/app/consumer.(*Consumer).handleMessages(0xc0004c4300, 0xf08c60, 0xc00054e630)
    /home/travis/gopath/src/github.com/jaegertracing/jaeger/cmd/ingester/app/consumer/consumer.go:124 +0x893
created by github.com/jaegertracing/jaeger/cmd/ingester/app/consumer.(*Consumer).Start.func1
    /home/travis/gopath/src/github.com/jaegertracing/jaeger/cmd/ingester/app/consumer/consumer.go:87 +0xbd
```

Gauges can, even though we have to keep an extra variable
around to keep count. In Prometheus Go library itself that
is not necessary as Gauge type provides `Inc` and `Dec`,
but Jaeger's wrapper does not have those exposed.

Fixes #1200.

Signed-off-by: Ivan Babrou <ibobrik@gmail.com>

Author: bobrik

cmd/ingester/app/consumer/consumer.go

@@ -46,9 +46,10 @@ type Consumer struct {
 
 	deadlockDetector deadlockDetector
 
-	partitionIDToState map[int32]*consumerState
-	partitionMapLock   sync.Mutex
-	partitionsHeld     metrics.Counter
+	partitionIDToState  map[int32]*consumerState
+	partitionMapLock    sync.Mutex
+	partitionsHeld      int64
+	partitionsHeldGauge metrics.Gauge
 }
 
 type consumerState struct {
@@ -60,13 +61,13 @@ type consumerState struct {
 func New(params Params) (*Consumer, error) {
 	deadlockDetector := newDeadlockDetector(params.MetricsFactory, params.Logger, params.DeadlockCheckInterval)
 	return &Consumer{
-		metricsFactory:     params.MetricsFactory,
-		logger:             params.Logger,
-		internalConsumer:   params.InternalConsumer,
-		processorFactory:   params.ProcessorFactory,
-		deadlockDetector:   deadlockDetector,
-		partitionIDToState: make(map[int32]*consumerState),
-		partitionsHeld:     partitionsHeld(params.MetricsFactory),
+		metricsFactory:      params.MetricsFactory,
+		logger:              params.Logger,
+		internalConsumer:    params.InternalConsumer,
+		processorFactory:    params.ProcessorFactory,
+		deadlockDetector:    deadlockDetector,
+		partitionIDToState:  make(map[int32]*consumerState),
+		partitionsHeldGauge: partitionsHeldGauge(params.MetricsFactory),
 	}, nil
 }
 
@@ -109,8 +110,12 @@ func (c *Consumer) Close() error {
 
 func (c *Consumer) handleMessages(pc sc.PartitionConsumer) {
 	c.logger.Info("Starting message handler", zap.Int32("partition", pc.Partition()))
-	c.partitionsHeld.Inc(1)
-	defer c.partitionsHeld.Inc(-1)
+	c.partitionsHeld++
+	c.partitionsHeldGauge.Update(c.partitionsHeld)
+	defer func() {
+		c.partitionsHeld--
+		c.partitionsHeldGauge.Update(c.partitionsHeld)
+	}()
 	c.partitionMapLock.Lock()
 	wg := &c.partitionIDToState[pc.Partition()].wg
 	c.partitionMapLock.Unlock()

cmd/ingester/app/consumer/consumer_metrics.go

@@ -61,6 +61,6 @@ func (c *Consumer) partitionMetrics(partition int32) partitionMetrics {
 		startCounter: f.Counter(metrics.Options{Name: "partition-start", Tags: nil})}
 }
 
-func partitionsHeld(metricsFactory metrics.Factory) metrics.Counter {
-	return metricsFactory.Namespace(metrics.NSOptions{Name: consumerNamespace, Tags: nil}).Counter(metrics.Options{Name: "partitions-held", Tags: nil})
+func partitionsHeldGauge(metricsFactory metrics.Factory) metrics.Gauge {
+	return metricsFactory.Namespace(metrics.NSOptions{Name: consumerNamespace, Tags: nil}).Gauge(metrics.Options{Name: "partitions-held", Tags: nil})
 }

cmd/ingester/app/consumer/consumer_test.go

@@ -90,12 +90,12 @@ func newConsumer(
 
 	logger, _ := zap.NewDevelopment()
 	return &Consumer{
-		metricsFactory:     metricsFactory,
-		logger:             logger,
-		internalConsumer:   consumer,
-		partitionIDToState: make(map[int32]*consumerState),
-		partitionsHeld:     partitionsHeld(metricsFactory),
-		deadlockDetector:   newDeadlockDetector(metricsFactory, logger, time.Second),
+		metricsFactory:      metricsFactory,
+		logger:              logger,
+		internalConsumer:    consumer,
+		partitionIDToState:  make(map[int32]*consumerState),
+		partitionsHeldGauge: partitionsHeldGauge(metricsFactory),
+		deadlockDetector:    newDeadlockDetector(metricsFactory, logger, time.Second),
 
 		processorFactory: ProcessorFactory{
 			topic:          topic,