fix: remove initial global HTTP client usage (#20128)

This PR makes the initial steps at removing usage of the global Go HTTP
client, which was seen to have impacts on test flakiness in
https://github.com/coder/internal/issues/1020. The first commit removes
uses from tests, with the exception of one test that is tightly coupled
to the default client. The second commit makes easy/low-risk removals
from application code. This should have some impact to reduce test flakiness.

enterprise/cli/provisionerdaemonstart_test.go

@@ -495,6 +495,7 @@ func TestProvisionerDaemon_PrometheusEnabled(t *testing.T) {
 	// Fetch metrics from Prometheus endpoint
 	var req *http.Request
 	var res *http.Response
+	httpClient := &http.Client{}
 	require.Eventually(t, func() bool {
 		req, err = http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("http://127.0.0.1:%d", prometheusPort), nil)
 		if err != nil {
@@ -503,7 +504,7 @@ func TestProvisionerDaemon_PrometheusEnabled(t *testing.T) {
 		}
 
 		// nolint:bodyclose
-		res, err = http.DefaultClient.Do(req)
+		res, err = httpClient.Do(req)
 		if err != nil {
 			t.Logf("unable to call Prometheus endpoint: %s", err.Error())
 			return false

enterprise/cli/proxyserver_test.go

@@ -114,11 +114,12 @@ func TestWorkspaceProxy_Server_PrometheusEnabled(t *testing.T) {
 
 	// Fetch metrics from Prometheus endpoint
 	var res *http.Response
+	client := &http.Client{}
 	require.Eventually(t, func() bool {
 		req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("http://127.0.0.1:%d", prometheusPort), nil)
 		assert.NoError(t, err)
 		// nolint:bodyclose
-		res, err = http.DefaultClient.Do(req)
+		res, err = client.Do(req)
 		return err == nil
 	}, testutil.WaitShort, testutil.IntervalFast)
 	defer res.Body.Close()

enterprise/cli/server_test.go

@@ -43,13 +43,14 @@ func TestServer_Single(t *testing.T) {
 	)
 	clitest.Start(t, inv.WithContext(ctx))
 	accessURL := waitAccessURL(t, cfg)
+	client := &http.Client{}
 	require.Eventually(t, func() bool {
 		reqCtx := testutil.Context(t, testutil.IntervalMedium)
 		req, err := http.NewRequestWithContext(reqCtx, http.MethodGet, accessURL.String()+"/healthz", nil)
 		if err != nil {
 			panic(err)
 		}
-		resp, err := http.DefaultClient.Do(req)
+		resp, err := client.Do(req)
 		if err != nil {
 			t.Log("/healthz not ready yet")
 			return false