Opentracing support

vendor/github.com/openzipkin/zipkin-go-opentracing/flag/flags.go

@@ -0,0 +1,39 @@
+package flag
+
+// Flags provides the ability to create and communicate feature flags.
+type Flags uint64
+
+// Flags is a bitset
+const (
+	Debug Flags = 1 << 0
+
+	// All flags below deal with binaryPropagators. They will be discarded in the
+	// textMapPropagator (not read and not set)
+
+	// SamplingSet and Sampled handle Sampled tribool logic for interop with
+	// instrumenting libraries / propagation channels not using a separate Sampled
+	// header and potentially encoding this in flags.
+	//
+	// When we receive a flag we do this:
+	// 1. Sampled bit is set => true
+	// 2. Sampled bit is not set => inspect SamplingSet bit.
+	// 2a. SamplingSet bit is set => false
+	// 2b. SamplingSet bit is not set => null
+	// Note on 2b.: depending on the propagator having a separate Sampled header
+	// we either assume Sampling is false or unknown. In the latter case we will
+	// run our sampler even though we are not the root of the trace.
+	//
+	// When propagating to a downstream service we will always be explicit and
+	// will provide a set SamplingSet bit in case of our binary propagator either
+	SamplingSet Flags = 1 << 1
+	Sampled     Flags = 1 << 2
+	// When set, we can ignore the value of the parentId. This is used for binary
+	// fixed width transports or transports like proto3 that return a default
+	// value if a value has not been set (thus not enabling you to distinguish
+	// between the value being set to the default or not set at all).
+	//
+	// While many zipkin systems re-use a trace id as the root span id, we know
+	// that some don't. With this flag, we can tell for sure if the span is root
+	// as opposed to the convention trace id == span id == parent id.
+	IsRoot Flags = 1 << 3
+)