Change basic stats to use opentracing rather than Prometheus API (#671)

* Change basic stats to use opentracing rather than Prometheus API directly

* Just ran gofmt

* Extract opentracing access for metrics to common/metrics.go

* Replace quotes strings with constants where possible

api/common/metrics.go

@@ -0,0 +1,101 @@
+package common
+
+import (
+	"context"
+	"github.com/opentracing/opentracing-go"
+	"github.com/opentracing/opentracing-go/log"
+)
+
+// IncrementGauge increments the specified gauge metric
+// It does this by logging an appropriate field value to a tracing span.
+func IncrementGauge(ctx context.Context, metric string) {
+	// The field name we use is the specified metric name prepended with FieldnamePrefixGauge to designate that it is a Prometheus gauge metric
+	// The collector will replace that prefix with "fn_" and use the result as the Prometheus metric name.
+	fieldname := FieldnamePrefixGauge + metric
+
+	// Spans are not processed by the collector until the span ends, so to prevent any delay
+	// in processing the stats when the current span is long-lived we create a new span for every call
+	// suffix the span name with SpannameSuffixDummy to denote that it is used only to hold a metric and isn't itself of any interest
+	span, ctx := opentracing.StartSpanFromContext(ctx, fieldname+SpannameSuffixDummy)
+	defer span.Finish()
+
+	// gauge metrics are actually float64; here we log that it should be increased by +1
+	span.LogFields(log.Float64(fieldname, 1.))
+}
+
+// DecrementGauge decrements the specified gauge metric
+// It does this by logging an appropriate field value to a tracing span.
+func DecrementGauge(ctx context.Context, metric string) {
+	// The field name we use is the specified metric name prepended with FieldnamePrefixGauge to designate that it is a Prometheus gauge metric
+	// The collector will replace that prefix with "fn_" and use the result as the Prometheus metric name.
+	fieldname := FieldnamePrefixGauge + metric
+
+	// Spans are not processed by the collector until the span ends, so to prevent any delay
+	// in processing the stats when the current span is long-lived we create a new span for every call.
+	// suffix the span name with SpannameSuffixDummy to denote that it is used only to hold a metric and isn't itself of any interest
+	span, ctx := opentracing.StartSpanFromContext(ctx, fieldname+SpannameSuffixDummy)
+	defer span.Finish()
+
+	// gauge metrics are actually float64; here we log that it should be increased by -1
+	span.LogFields(log.Float64(fieldname, -1.))
+}
+
+// IncrementCounter increments the specified counter metric
+// It does this by logging an appropriate field value to a tracing span.
+func IncrementCounter(ctx context.Context, metric string) {
+	// The field name we use is the specified metric name prepended with FieldnamePrefixCounter to designate that it is a Prometheus counter metric
+	// The collector will replace that prefix with "fn_" and use the result as the Prometheus metric name.
+	fieldname := FieldnamePrefixCounter + metric
+
+	// Spans are not processed by the collector until the span ends, so to prevent any delay
+	// in processing the stats when the current span is long-lived we create a new span for every call.
+	// suffix the span name with SpannameSuffixDummy to denote that it is used only to hold a metric and isn't itself of any interest
+	span, ctx := opentracing.StartSpanFromContext(ctx, fieldname+SpannameSuffixDummy)
+	defer span.Finish()
+
+	// counter metrics are actually float64; here we log that it should be increased by +1
+	span.LogFields(log.Float64(fieldname, 1.))
+}
+
+// If required, create a scalar version of PublishHistograms that publishes a single histogram metric
+
+// PublishHistograms publishes the specifed histogram metrics
+// It does this by logging appropriate field values to a tracing span
+func PublishHistograms(ctx context.Context, metrics map[string]float64) {
+
+	// Spans are not processed by the collector until the span ends, so to prevent any delay
+	// in processing the stats when the current span is long-lived we create a new span for every call.
+	// suffix the span name with SpannameSuffixDummy to denote that it is used only to hold a metric and isn't itself of any interest
+	span, ctx := opentracing.StartSpanFromContext(ctx, "histogram_metrics"+SpannameSuffixDummy)
+	defer span.Finish()
+
+	for key, value := range metrics {
+		// The field name we use is the metric name prepended with FieldnamePrefixHistogram to designate that it is a Prometheus histogram metric
+		// The collector will replace that prefix with "fn_" and use the result as the Prometheus metric name.
+		fieldname := FieldnamePrefixHistogram + key
+		span.LogFields(log.Float64(fieldname, value))
+	}
+}
+
+const (
+
+	// FnPrefix is a constant for "fn_", used as a prefix for span names, field names, Prometheus metric names and Prometheus label names
+	FnPrefix = "fn_"
+
+	// FieldnamePrefixHistogram is prefixed to the name of a logged field
+	// to denote that it corresponds to a histogram metric
+	FieldnamePrefixHistogram = FnPrefix + "histogram_"
+
+	// FieldnamePrefixCounter is prefixed to the name of a logged field
+	// to denote that it corresponds to a counter metric
+	FieldnamePrefixCounter = FnPrefix + "counter_"
+
+	// FieldnamePrefixGauge is prefixed to the name of a logged field
+	// to denote that it corresponds to a gauge metric
+	FieldnamePrefixGauge = FnPrefix + "gauge_"
+
+	// SpannameSuffixDummy is suffixed to the name of a tracing span
+	// to denote that it has been created solely for the purpose of carrying metric values
+	// and is not itself of any interest and should not be converted to a Prometheus duration metric
+	SpannameSuffixDummy = "_dummy"
+)