Supporting perf events with opentracing

Perf events can provide additional insights into what's happening within
a span. Although, latency of a span can help us in identifying the
bottle necks within a trace, perf events can compliment that in
identifying as to where was the bottle neck and which platform resource
was busy for that span. This patch is in addition to the perf events
support to zipkin-go-opentracing.

Signed-off-by: Hemant Kumar <hemant@linux.vnet.ibm.com>

tracer.go

@@ -145,6 +145,10 @@ type StartSpanOptions struct {
 	// If specified, the caller hands off ownership of Tags at
 	// StartSpan() invocation time.
 	Tags map[string]interface{}
+
+	// Perfevent is the platform metric's name or an event name supported
+	// by perf.
+	Perfevent string
 }
 
 // StartSpanOption instances (zero or more) may be passed to Tracer.StartSpan.
@@ -262,6 +266,14 @@ func (t StartTime) Apply(o *StartSpanOptions) {
 	o.StartTime = time.Time(t)
 }
 
+// PerfString is the perf event's name.
+type PerfString string
+
+// Apply satisfies the StartSpanOption interface
+func (e PerfString) Apply(o *StartSpanOptions) {
+	o.Perfevent = string(e)
+}
+
 // Tags are a generic map from an arbitrary string key to an opaque value type.
 // The underlying tracing system is responsible for interpreting and
 // serializing the values.