Report the build and pod name each proxy process is running

[liam-lowe]

Adds a build_info gauge, always 1, labelled with the version the running binary was stamped with.

collectors.NewBuildInfoCollector reports the Go module version. That is not the same thing. This reports the -X main.Version stamp. A half-finished rollout is therefore visible as two series rather than one.

Reporting it needs the process to know who it is. This therefore also adds Identity: a Version and a MemberID, supplied to NewProxy rather than read from config. A value in the shared config file would be identical on every replica. That is the one thing an identity cannot be.

DefaultIdentity reads $POD_NAME first. A Kubernetes deployment can therefore be explicit. The fallback is the hostname. Inside a pod that is the pod name. The chart supplies POD_NAME through the downward API.

NewProxy gains a third parameter. Every caller is updated. app/app.go supplies DefaultIdentity. The test callers supply a literal.

Testing

• make lint: 0 issues.
• go test -race ./...: every package passes.
• helm unittest s2s-proxy/: passes. make helm-example shows no drift.
• No new test. build_info is set on a path the existing NewProxy tests already exercise. DefaultIdentity reads only the environment. Both are worth covering when a consumer of MemberID arrives.