Context propagation with B3 Headers + Gin about go-agent

[kevinmichaelchen]

I just discovered InsertDistributedTraceHeaders and AcceptDistributedTraceHeaders.

Should be possible to write some Gin middleware that parses B3 headers and "passes them in" as W3C headers:

// NewB3Handler creates a Gin middleware that links New Relic transactions by
// accepting distributed trace headers from another transaction.
func NewB3Handler() gin.HandlerFunc {
  return func(c *gin.Context) {
    txn := nrgin.Transaction(c)

    if txn != nil {
      traceID := c.GetHeader("X-B3-TraceId")
      spanID := c.GetHeader("X-B3-SpanId")
      sampled := c.GetHeader("X-B3-Sampled")

      // w3c spec uses hex encoding: https://www.w3.org/TR/trace-context/#trace-flags
      sampledHex := "00"
      if sampled == "1" || sampled == "true" {
        sampledHex = "01"
      }

      // https://www.w3.org/TR/trace-context/#traceparent-header
      w3cTraceParent := fmt.Sprintf("00-%s-%s-%s", traceID, spanID, sampledHex)

      // https://docs.newrelic.com/docs/distributed-tracing/concepts/how-new-relic-distributed-tracing-works/#headers
      var hdrs http.Header = map[string][]string{
        newrelic.DistributedTraceW3CTraceParentHeader: []string{w3cTraceParent},
        // Leaving the tracestate header blank for now.
        // Per the official W3C spec: failure to parse tracestate MUST NOT affect the parsing of traceparent.
        // https://github.com/newrelic/go-agent/blob/v3.20.1/v3/newrelic/distributed_tracing_test.go#L249-L277
        newrelic.DistributedTraceW3CTraceStateHeader: []string{""},
        newrelic.DistributedTraceNewRelicHeader:      []string{c.GetHeader(newrelic.DistributedTraceNewRelicHeader)},
      }

      // Links transactions by accepting distributed trace headers from
      // another transaction.
      txn.AcceptDistributedTraceHeaders(newrelic.TransportHTTP, hdrs)
      txn.AcceptDistributedTraceHeaders(newrelic.TransportHTTPS, hdrs)
    }

    c.Next()
  }
}