Add otelhttp.Middleware for better compatiblity with third-party HTTP…

… routers and middleware chainers

The key difference between the existing otelhttp.Handler and the new
otelhttp.Middleware is that `Middleware` takes the `next` handler as an
argument after construction, wheres the existing `Handler` works by
wrapping one specific http.Handler at construction time.

CHANGELOG.md

@@ -10,6 +10,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Added
 
+- An HTTP middleware in addition to the existing HTTP handler. The middleware is constructed once and afterwards can take different handlers as arguments, whereas the handler wraps a single http.Handler during construction. This allows better compatibility with packages like github.com/urfave/negroni and github.com/go-chi/chi.
 - AWS SDK add `rpc.system` attribute in `go.opentelemetry.io/contrib/instrumentation/github.com/aws/aws-sdk-go-v2/otelaws`. (#3582, #3617)
 - Add the new `go.opentelemetry.io/contrib/instrgen` package to provide auto-generated source code instrumentation. (#3068, #3108)
 

instrumentation/net/http/otelhttp/handler.go

@@ -31,16 +31,10 @@ import (
 	"go.opentelemetry.io/otel/trace"
 )
 
-var _ http.Handler = &Handler{}
-
-// Handler is http middleware that corresponds to the http.Handler interface and
-// is designed to wrap a http.Mux (or equivalent), while individual routes on
-// the mux are wrapped with WithRouteTag. A Handler will add various attributes
-// to the span using the attribute.Keys defined in this package.
-type Handler struct {
+// middleware is an http middleware which wraps the next handler in a span.
+type middleware struct {
 	operation string
 	server    string
-	handler   http.Handler
 
 	tracer            trace.Tracer
 	meter             metric.Meter
@@ -60,11 +54,16 @@ func defaultHandlerFormatter(operation string, _ *http.Request) string {
 	return operation
 }
 
-// NewHandler wraps the passed handler, functioning like middleware, in a span
-// named after the operation and with any provided Options.
+// NewHandler wraps the passed handler in a span named after the operation and
+// with any provided Options.
 func NewHandler(handler http.Handler, operation string, opts ...Option) http.Handler {
-	h := Handler{
-		handler:   handler,
+	return NewMiddleware(operation, opts...)(handler)
+}
+
+// / NewMiddleware returns a tracing middleware from the given operation name and
+// options.
+func NewMiddleware(operation string, opts ...Option) func(http.Handler) http.Handler {
+	h := middleware{
 		operation: operation,
 	}
 
@@ -77,10 +76,14 @@ func NewHandler(handler http.Handler, operation string, opts ...Option) http.Han
 	h.configure(c)
 	h.createMeasures()
 
-	return &h
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			h.serveHTTP(w, r, next)
+		})
+	}
 }
 
-func (h *Handler) configure(c *config) {
+func (h *middleware) configure(c *config) {
 	h.tracer = c.Tracer
 	h.meter = c.Meter
 	h.propagators = c.Propagators
@@ -100,7 +103,7 @@ func handleErr(err error) {
 	}
 }
 
-func (h *Handler) createMeasures() {
+func (h *middleware) createMeasures() {
 	h.counters = make(map[string]instrument.Int64Counter)
 	h.valueRecorders = make(map[string]instrument.Float64Histogram)
 
@@ -118,13 +121,14 @@ func (h *Handler) createMeasures() {
 	h.valueRecorders[ServerLatency] = serverLatencyMeasure
 }
 
-// ServeHTTP serves HTTP requests (http.Handler).
-func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+// serveHTTP sets up tracing and calls the given next http.Handler with the span
+// context injected into the request context.
+func (h *middleware) serveHTTP(w http.ResponseWriter, r *http.Request, next http.Handler) {
 	requestStartTime := time.Now()
 	for _, f := range h.filters {
 		if !f(r) {
 			// Simply pass through to the handler if a filter rejects the request
-			h.handler.ServeHTTP(w, r)
+			next.ServeHTTP(w, r)
 			return
 		}
 	}
@@ -210,7 +214,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	labeler := &Labeler{}
 	ctx = injectLabeler(ctx, labeler)
 
-	h.handler.ServeHTTP(w, r.WithContext(ctx))
+	next.ServeHTTP(w, r.WithContext(ctx))
 
 	setAfterServeAttributes(span, bw.read, rww.written, rww.statusCode, bw.err, rww.err)