Instrumentation: Proxy status code correction and various improvements (#47473)

For a proxied request, e.g. Grafana's datasource or plugin proxy:
If the request is cancelled, e.g. from the browser, the HTTP status code is 
now 499 Client closed request instead of 502 Bad gateway.
If the request times out, e.g. takes longer time than allowed, the HTTP status 
code is now 504 Gateway timeout instead of 502 Bad gateway.
This also means that request metrics and logs will get their status codes 
adjusted according to above.

Fixes #46337
Fixes #46338

pkg/api/pluginproxy/ds_proxy_test.go

@@ -456,8 +456,6 @@ func TestDataSourceProxy_routeRule(t *testing.T) {
 
 		assert.Equal(t, "http://host/root/path/to/folder/", req.URL.String())
 
-		assert.Empty(t, req.Header.Get("Origin"))
-		assert.Empty(t, req.Header.Get("Referer"))
 		assert.Equal(t, "stillthere", req.Header.Get("X-Canary"))
 	})