Add example for gorilla mux instrumentation

plugins/gorilla/mux/example/Dockerfile

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+# Copyright The OpenTelemetry Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+FROM golang:alpine AS base
+COPY . /src/
+WORKDIR /src/plugins/gorilla/mux
+
+FROM base AS mux-server
+RUN go install ./example/server.go
+CMD ["/go/bin/server"]

plugins/gorilla/mux/example/README.md

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+# mux-gonic instrumentation example
+
+An HTTP server using gorilla mux and instrumentation. The server has a
+`/users/{id:[0-9]+}` endpoint. The server generates span information to
+`stdout`.
+
+These instructions expect you have
+[docker-compose](https://docs.docker.com/compose/) installed.
+
+Bring up the `mux-server` and `mux-client` services to run the
+example:
+
+```sh
+docker-compose up --detach mux-server mux-client
+```
+
+The `mux-client` service sends just one HTTP request to `mux-server`
+and then exits. View the span generated by `mux-server` in the logs:
+
+```sh
+docker-compose logs mux-server
+```
+
+Shut down the services when you are finished with the example:
+
+```sh
+docker-compose down
+```

plugins/gorilla/mux/example/docker-compose.yml

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+# Copyright The OpenTelemetry Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+version: "3.7"
+services:
+  mux-client:
+    image: golang:alpine
+    networks:
+      - example
+    command:
+      - "/bin/sh"
+      - "-c"
+      - "wget http://mux-server:8080/users/123 && cat 123"
+    depends_on:
+      - mux-server
+  mux-server:
+    build:
+      dockerfile: $PWD/Dockerfile
+      context: ../../../..
+    ports:
+      - "8080:80"
+    command:
+      - "/bin/sh"
+      - "-c"
+      - "/go/bin/server"
+    networks:
+      - example
+networks:
+  example:

plugins/gorilla/mux/example/server.go

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+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net/http"
+
+	"github.com/gorilla/mux"
+
+	muxtrace "go.opentelemetry.io/contrib/plugins/gorilla/mux"
+	otelglobal "go.opentelemetry.io/otel/api/global"
+	otelkey "go.opentelemetry.io/otel/api/key"
+	oteltrace "go.opentelemetry.io/otel/api/trace"
+	oteltracestdout "go.opentelemetry.io/otel/exporters/trace/stdout"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+var tracer = otelglobal.Tracer("mux-server")
+
+func main() {
+	initTracer()
+	r := mux.NewRouter()
+	r.Use(muxtrace.Middleware("my-server"))
+	r.HandleFunc("/users/{id:[0-9]+}", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		vars := mux.Vars(r)
+		id := vars["id"]
+		name := getUser(r.Context(), id)
+		reply := fmt.Sprintf("user %s (id %s)\n", name, id)
+		_, _ = w.Write(([]byte)(reply))
+	}))
+	http.Handle("/", r)
+	_ = http.ListenAndServe(":8080", nil)
+}
+
+func initTracer() {
+	exporter, err := oteltracestdout.NewExporter(oteltracestdout.Options{PrettyPrint: true})
+	if err != nil {
+		log.Fatal(err)
+	}
+	cfg := sdktrace.Config{
+		DefaultSampler: sdktrace.AlwaysSample(),
+	}
+	tp, err := sdktrace.NewProvider(
+		sdktrace.WithConfig(cfg),
+		sdktrace.WithSyncer(exporter),
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+	otelglobal.SetTraceProvider(tp)
+}
+
+func getUser(ctx context.Context, id string) string {
+	_, span := tracer.Start(ctx, "getUser", oteltrace.WithAttributes(otelkey.String("id", id)))
+	defer span.End()
+	if id == "123" {
+		return "muxtrace tester"
+	}
+	return "unknown"
+}