Add client.TimedClient implementing Requester (#74)

* Add `client.TimedClient` implementing `Requester`

This helps in chaining clients doing requests. Such as

    client.NewTimedClient(http.DefaultClient, someCollector}

* Code cleanup, add tests

- Return concrete type
- Use custom type for context key
- Add tests

Author: rndstr

http/client/client.go

@@ -16,6 +16,39 @@ type Requester interface {
 	Do(req *http.Request) (*http.Response, error)
 }
 
+// TimedClient instruments a request. It implements Requester.
+type TimedClient struct {
+	client    Requester
+	collector instrument.Collector
+}
+
+type contextKey int
+
+// OperationNameContextKey specifies the operation name location within the context
+// for instrumentation.
+const OperationNameContextKey contextKey = 0
+
+// NewTimedClient creates a Requester that instruments requests on `client`.
+func NewTimedClient(client Requester, collector instrument.Collector) *TimedClient {
+	return &TimedClient{
+		client:    client,
+		collector: collector,
+	}
+}
+
+// Do executes the request.
+func (c TimedClient) Do(r *http.Request) (*http.Response, error) {
+	return TimeRequest(r.Context(), c.operationName(r), c.collector, c.client, r)
+}
+
+func (c TimedClient) operationName(r *http.Request) string {
+	operation, _ := r.Context().Value(OperationNameContextKey).(string)
+	if operation == "" {
+		operation = r.URL.Path
+	}
+	return operation
+}
+
 // TimeRequest performs an HTTP client request and records the duration in a histogram.
 func TimeRequest(ctx context.Context, operation string, coll instrument.Collector, client Requester, request *http.Request) (*http.Response, error) {
 	var response *http.Response

http/client/client_test.go

@@ -0,0 +1,29 @@
+package client
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTimedClient_operationName(t *testing.T) {
+	r, err := http.NewRequest("GET", "https://weave.test", nil)
+	assert.NoError(t, err)
+
+	r = r.WithContext(context.WithValue(context.Background(), OperationNameContextKey, "opp"))
+	c := NewTimedClient(http.DefaultClient, nil)
+
+	assert.Equal(t, "opp", c.operationName(r))
+}
+
+func TestTimedClient_operationName_Default(t *testing.T) {
+	r, err := http.NewRequest("GET", "https://weave.test/you/know/me", nil)
+	assert.NoError(t, err)
+
+	r = r.WithContext(context.Background())
+	c := NewTimedClient(http.DefaultClient, nil)
+
+	assert.Equal(t, "/you/know/me", c.operationName(r))
+}