Add a virtual cluster stat for when a downstream stream termination occurs

[Augustyniak]

Description: Add a virtual cluster stat that's emitted when a downstream stream is terminated before a given stream (request) completes. This is important from the perspective of Envoy Mobile and mobile clients in general. In mobile environment it a request - an Envoy downstream stream - can be started and terminated soon after (before the stream has a chance to finish) as a result of certain user's interaction in the mobile app.

Currently, virtual cluster's stats do not capture information about the termination of such request. When a request starts, upstream_rq_total stat is incremented but there is not corresponding "request completion" stat that gets incremented when a downstream stream is terminated.

Some of the ideas for the proposed stat name (ordered list):

• upstream_rq_dropped
• upstream_rq_destroyed
• upstream_rq_canceled

Relevant Links: The list of currently available virtual cluster stats can be found at https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/router_filter#virtual-clusters.

[phlax]

cc @jmarantz

[github-actions[bot]]

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