Add otelhttp.NewMiddleware for better compatiblity with third-party H…

…TTP routers and middleware chainers

The key difference between the existing otelhttp.NewHandler and
otelhttp.NewMiddleware is that the middleware takes the `next` handler
as an argument after construction, wheres NewHandler 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
 
+- Add `NewMiddleware` function in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp`. (#2964)
 - 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)
 
@@ -24,6 +25,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Prevent taking from reservoir in AWS XRay Remote Sampler when there is zero capacity in `go.opentelemetry.io/contrib/samplers/aws/xray`. (#3684)
 - Fix `otelhttp.Handler` in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp` to propagate multiple `WriteHeader` calls while persisting the initial `statusCode`. (#3580)
 
+### Removed
+
+- Remove `Handler` type in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp`. (#2964)
+
 ## [1.16.0-rc.2/0.41.0-rc.2/0.10.0-rc.2] - 2023-03-23
 
 ### Added

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
+// enriches it with metrics.
 func NewHandler(handler http.Handler, operation string, opts ...Option) http.Handler {
-	h := Handler{
-		handler:   handler,
+	return NewMiddleware(operation, opts...)(handler)
+}
+
+// NewMiddleware returns a tracing and metrics 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)