[release-1.4] Fill response flag for stats plugin

[istio-testing]

This is an automated cherry-pick of #2527

[bianpengyuan]

@istio/release-managers-1-4 Any help on getting this in?

[bianpengyuan]

--- FAIL: TestStackdriverParallel (16.08s)
    stackdriver_xds_test.go:301: Get http://127.0.0.1:20183: net/http: request canceled (Client.Timeout exceeded while awaiting headers)
FAIL
FAIL	istio.io/proxy/test/envoye2e/stackdriver_plugin	75.415s
?   	istio.io/proxy/test/envoye2e/stackdriver_plugin/fake_stackdriver	[no test files]
ok  	istio.io/proxy/test/envoye2e/tcp_metadata_exchange	4.052s
Makefile.core.mk:77: recipe for target 'test_tsan' failed

This is a known flake/issue that envoy keeps drained listener for up to 45s + frequent listener updates in the test. Retry..

[bianpengyuan]

/test proxy-presubmit-tsan

[kyessenov]

I tuned drainage so that zombie listeners don't fail the test. This failure was due to TSAN violation that looks like the same one I filed in envoy today.

[kyessenov]

/test proxy-presubmit-tsan

If it keeps failing, we may need to force skip it in TSAN.