balancer/pickfirstleaf: Avoid reading Address.Metadata

[arjan-bal]

Addresses: #8225

This PR makes the new pickfirst ignore the deprecated Metadata field. This is required since pickfirst is currently using the Medata field to lookup addresses in an addressList and not using the Metadata while looking up addresses from the AddressMap. This change makes the behaviour of both the data structures consistent.

RELEASE NOTES:

• balancer/pickfirstleaf: Ignore the deprecated Address.Metadata field.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.04%. Comparing base (ce35fd4) to head (c290057).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8227      +/-   ##
==========================================
- Coverage   82.16%   82.04%   -0.12%     
==========================================
  Files         410      412       +2     
  Lines       40248    40487     +239     
==========================================
+ Hits        33068    33219     +151     
- Misses       5830     5891      +61     
- Partials     1350     1377      +27     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	86.57% <100.00%> (-0.03%)	⬇️

... and 57 files with indirect coverage changes

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[easwars]

Do we still want to do this?

The most recent comment on etcd-io/etcd#19700 (comment) says that etcd itself is not affected by this and that they are going to prioritize the long term fix of not using the Metadata field over any short term fixes.

But I also see that your note saying pickfirstleaf is affected and that it does not work with Addresses that contain the Metadata field. Can we say that the new pick_first policy will not support the Metadata field since it has now been marked as deprecated for over 5 years and thereby simplify our lives?

[arjan-bal]

Waiting to discuss this with @dfawley. If we think is fine not to support Metadata to differentiate addresses in the new pickfirst, we would need to change the following condition. Having a test for the behaviour would be good.

grpc-go/balancer/pickfirst/pickfirstleaf/pickfirstleaf.go

Lines 924 to 928 in 732f3f3

	func equalAddressIgnoringBalAttributes(a, b *resolver.Address) bool {
	return a.Addr == b.Addr && a.ServerName == b.ServerName &&
	a.Attributes.Equal(b.Attributes) &&
	a.Metadata == b.Metadata
	}

[ahrtr]

The most recent comment on etcd-io/etcd#19700 (comment) says that etcd itself

etcd isn't affected, but etcd's users are affected. I think we still need the fix regardless the long-term plan (which is not yet finalized).