Add support for tracing via OpenCencus

This adds a few flags for configuring the tracer.
Includes support for jaeger tracing (built into OC).

cmd/census.go

@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"github.com/pkg/errors"
+	"go.opencensus.io/trace"
+)
+
+var (
+	tracingExporters = make(map[string]TracingExporterInitFunc)
+
+	reservedTagNames = map[string]bool{
+		"operatingSystem": true,
+		"provider":        true,
+		"nodeName":        true,
+	}
+)
+
+// TracingExporterOptions is used to pass options to the configured tracer
+type TracingExporterOptions struct {
+	Tags        map[string]string
+	ServiceName string
+}
+
+// TracingExporterInitFunc is the function that is called to initialize an exporter.
+// This is used when registering an exporter and called when a user specifed they want to use the exporter.
+type TracingExporterInitFunc func(TracingExporterOptions) (trace.Exporter, error)
+
+// RegisterTracingExporter registers a tracing exporter.
+// For a user to select an exporter, it must be registered here.
+func RegisterTracingExporter(name string, f TracingExporterInitFunc) {
+	tracingExporters[name] = f
+}
+
+// GetTracingExporter gets the specified tracing exporter passing in the options to the exporter init function.
+// For an exporter to be availbale here it must be registered with `RegisterTracingExporter`.
+func GetTracingExporter(name string, opts TracingExporterOptions) (trace.Exporter, error) {
+	f, ok := tracingExporters[name]
+	if !ok {
+		return nil, errors.Errorf("tracing exporter %q not found", name)
+	}
+	return f(opts)
+}
+
+// AvailableTraceExporters gets the list of registered exporters
+func AvailableTraceExporters() []string {
+	out := make([]string, 0, len(tracingExporters))
+	for k := range tracingExporters {
+		out = append(out, k)
+	}
+	return out
+}