Fix configuration merging for implicit tproxy upstreams.
Change the merging logic so that the wildcard upstream has correct proxy-defaults
and service-defaults values combined into it. It did not previously merge all fields,
and the wildcard upstream did not exist unless service-defaults existed (it ignored
proxy-defaults, essentially).
Change the way we fetch upstream configuration in the xDS layer so that it falls back
to the wildcard when no matching upstream is found. This is what allows implicit peer
upstreams to have the correct "merged" config.
Change proxycfg to always watch local mesh gateway endpoints whenever a peer upstream
is found. This simplifies the logic so that we do not have to inspect the "merged"
configuration on peer upstreams to extract the mesh gateway mode.
Transparent proxies can set up filter chains that allow direct
connections to upstream service instances. Services that can be dialed
directly are stored in the PassthroughUpstreams map of the proxycfg
snapshot.
Previously these addresses were not being cleaned up based on new
service health data. The list of addresses associated with an upstream
service would only ever grow.
As services scale up and down, eventually they will have instances
assigned to an IP that was previously assigned to a different service.
When IP addresses are duplicated across filter chain match rules the
listener config will be rejected by Envoy.
This commit updates the proxycfg snapshot management so that passthrough
addresses can get cleaned up when no longer associated with a given
upstream.
There is still the possibility of a race condition here where due to
timing an address is shared between multiple passthrough upstreams.
That concern is mitigated by #12195, but will be further addressed
in a follow-up.
The gist here is that now we use a value-type struct proxycfg.UpstreamID
as the map key in ConfigSnapshot maps where we used to use "upstream
id-ish" strings. These are internal only and used just for bidirectional
trips through the agent cache keyspace (like the discovery chain target
struct).
For the few places where the upstream id needs to be projected into xDS,
that's what (proxycfg.UpstreamID).EnvoyID() is for. This lets us ALWAYS
inject the partition and namespace into these things without making
stuff like the golden testdata diverge.
