(wip) stackdriver: add cluster name labels

[douglas-reid]

Adds support for (source|destination)_cluster_name labels to Stackdriver extension.

[istio-testing]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

[douglas-reid]

/test all

[douglas-reid]

/test all

[douglas-reid]

/test test_proxy

[douglas-reid]

/test test_proxy

[douglas-reid]

/test test_proxy

[ericvn]

/retest

[costinm]

Looks good as long as it is consistent, would appreciate a canonical doc with links to where the names come from.

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[douglas-reid]

/test test_proxy

[douglas-reid]

/test test_proxy

[douglas-reid]

/test all

[istio-testing]

@douglas-reid: PR needs rebase.

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