This adds support for the Incremental xDS protocol when using xDS v3. This is best reviewed commit-by-commit and will not be squashed when merged.
Union of all commit messages follows to give an overarching summary:
xds: exclusively support incremental xDS when using xDS v3
Attempts to use SoTW via v3 will fail, much like attempts to use incremental via v2 will fail.
Work around a strange older envoy behavior involving empty CDS responses over incremental xDS.
xds: various cleanups and refactors that don't strictly concern the addition of incremental xDS support
Dissolve the connectionInfo struct in favor of per-connection ResourceGenerators instead.
Do a better job of ensuring the xds code uses a well configured logger that accurately describes the connected client.
xds: pull out checkStreamACLs method in advance of a later commit
xds: rewrite SoTW xDS protocol tests to use protobufs rather than hand-rolled json strings
In the test we very lightly reuse some of the more boring protobuf construction helper code that is also technically under test. The important thing of the protocol tests is testing the protocol. The actual inputs and outputs are largely already handled by the xds golden output tests now so these protocol tests don't have to do double-duty.
This also updates the SoTW protocol test to exclusively use xDS v2 which is the only variant of SoTW that will be supported in Consul 1.10.
xds: default xds.Server.AuthCheckFrequency at use-time instead of construction-time
Allows setting -prometheus-backend-port to configure the cluster
envoy_prometheus_bind_addr points to.
Allows setting -prometheus-scrape-path to configure which path
envoy_prometheus_bind_addr exposes metrics on.
-prometheus-backend-port is used by the consul-k8s metrics merging feature, to
configure envoy_prometheus_bind_addr to point to the merged metrics
endpoint that combines Envoy and service metrics so that one set of
annotations on a Pod can scrape metrics from the service and it's Envoy
sidecar.
-prometheus-scrape-path is used to allow configurability of the path
where prometheus metrics are exposed on envoy_prometheus_bind_addr.
R.B. Boyer