Update B3 header names (#881)

* Update B3 header names

Correct the B3 single header name from `X-B3` to `b3`.

Use the lowercase version of the B3 multiple headers. This is based on
the fact that HTTP headers are case-insensitive, however, other
protocols may not be.

* Update Changelog

CHANGELOG.md

@@ -11,6 +11,11 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Changed
 
 - Update `CONTRIBUTING.md` to ask for updates to `CHANGELOG.md` with each pull request. (#879)
+- Use lowercase header names for B3 Multiple Headers. (#881)
+
+### Fixed
+
+- The B3 Single Header name is now correctly `b3` instead of the previous `X-B3`. (#881)
 
 ## [0.7.0] - 2020-06-26
 

api/trace/b3_propagator.go

@@ -23,13 +23,15 @@ import (
 )
 
 const (
-	B3SingleHeader       = "X-B3"
-	B3DebugFlagHeader    = "X-B3-Flags"
-	B3TraceIDHeader      = "X-B3-TraceId"
-	B3SpanIDHeader       = "X-B3-SpanId"
-	B3SampledHeader      = "X-B3-Sampled"
-	B3ParentSpanIDHeader = "X-B3-ParentSpanId"
-	b3TraceIDPadding     = "0000000000000000"
+	// Default B3 Header names.
+	B3SingleHeader       = "b3"
+	B3DebugFlagHeader    = "x-b3-flags"
+	B3TraceIDHeader      = "x-b3-traceid"
+	B3SpanIDHeader       = "x-b3-spanid"
+	B3SampledHeader      = "x-b3-sampled"
+	B3ParentSpanIDHeader = "x-b3-parentspanid"
+
+	b3TraceIDPadding = "0000000000000000"
 )
 
 // B3 propagator serializes SpanContext to/from B3 Headers.