tracing_keys.go
func (c tracingHeadersCarrier) Set(key, val string) {
	prefixedKey := tracingKeyEncoding.mapAndCache(key)
	c[prefixedKey] = val
}
func (c tracingHeadersCarrier) Set(key, val string) {
	prefixedKey := tracingKeyEncoding.mapAndCache(key)
	c[prefixedKey] = val
}
// Set implements Set() of opentracing.TextMapWriter