chore(cli): correctly report telemetry even when transport replaced (#7670)

By introducing the "ExtraHeaders" map, we can apply headers even when
handlers replace the transport, as in the case of our scaletests.

Also, only send telemetry header when it's small.

codersdk/client.go

@@ -85,8 +85,9 @@ var loggableMimeTypes = map[string]struct{}{
 // New creates a Coder client for the provided URL.
 func New(serverURL *url.URL) *Client {
 	return &Client{
-		URL:        serverURL,
-		HTTPClient: &http.Client{},
+		URL:          serverURL,
+		HTTPClient:   &http.Client{},
+		ExtraHeaders: make(http.Header),
 	}
 }
 
@@ -96,6 +97,9 @@ type Client struct {
 	mu           sync.RWMutex // Protects following.
 	sessionToken string
 
+	// ExtraHeaders are headers to add to every request.
+	ExtraHeaders http.Header
+
 	HTTPClient *http.Client
 	URL        *url.URL
 
@@ -189,6 +193,8 @@ func (c *Client) Request(ctx context.Context, method, path string, body interfac
 		return nil, xerrors.Errorf("create request: %w", err)
 	}
 
+	req.Header = c.ExtraHeaders.Clone()
+
 	tokenHeader := c.SessionTokenHeader
 	if tokenHeader == "" {
 		tokenHeader = SessionTokenHeader