feat(bigtable): add NewSessionClient constructor with session-dedicated channel pool

[sushanb]

Summary

Adds bigtable.SessionClient — a standalone, session-only handle that owns its own gRPC channel pool, stub, and ClientConfigurationManager, independent of any classic *Client the caller may also have open.

This is the first slice of the larger session-table-api plan: constructor + channel pool + config manager wiring only. Follow-ups will add per-resource SessionTable factory methods, cross-client caching on *Client, mixed-mode integration inside *Client, and sessionz debug forwarding.

Usage

sc, err := bigtable.NewSessionClient(ctx, "my-project", "my-instance",
    bigtable.ClientConfig{AppProfile: "default"})
if err != nil { … }
defer sc.Close()

mgr := sc.ConfigurationManager()  // subscribe to server-driven config

Notes for reviewers

• Option plumbing mirrors NewClientWithConfig verbatim so any behavioural drift between the two constructors stays visible in diff. When retry env vars, direct-access options, or feature-flag construction change in NewClientWithConfig, they should mirror here.
• First production caller of NewClientConfigurationManager. Every prior caller is in tests.
• Manager polling ctx — configManager.Start(context.Background()), not the constructor's ctx. The polling loop must outlive NewSessionClient's caller context; teardown flows through Close, not ctx cancellation.
• Close is idempotent via sync.Once. Ordering: manager → metrics tracer → channel pool, so no in-flight GetClientConfiguration lands on a closed pool.
• Reuses pingAndWarmChannelPrimer and pingAndWarmDirectAccessChecker for now. A session-specific noop primer and getClientConfigDirectAccessChecker (already anticipated in direct_access_checker.go:43-65) will land as follow-ups.

Explicitly out of scope (follow-ups)

• SessionClient.NewSessionTable* factory + SessionTable type
• sessionTables cache on *Client
• Mixed-mode composition (Client.sessionClient field, OpenTable routing)
• Session-specific noop ChannelPrimer
• getClientConfigDirectAccessChecker
• SessionDebug / ChannelDebug / ConfigDebug forwarding (waits on debugview package landing upstream)

Test plan

• go build ./... clean
• go vet ./... clean
• go test -run '^(TestNewSessionClient|TestSessionClient)' -count=1 passes
• go test -short ./internal/transport/ passes (no regression in the underlying pool machinery)
• Live smoke against vRPC sandbox — optional, for reviewer

/cc @mutianf

[gemini-code-assist]

Code Review

This pull request introduces SessionClient, a session-only handle to a Bigtable instance that manages its own gRPC channel pool, stub, and configuration manager, along with corresponding unit tests. The review feedback highlights several important improvements: ensuring proper cleanup of metricsTracerFactory if the constructor fails, making the background logger configurable instead of hardcoding log.Default(), adding a nil check for mPool in Close() to prevent potential panics, and deferring the cleanup function in the idempotency test to avoid leaking the in-memory test server.

[gemini-code-assist]

When initializing resources that require cleanup (such as metrics) inside a constructor, use named return errors and a defer block to clean up the resources if the constructor fails. This ensures that if any subsequent step returns an error, the newly created metricsTracerFactory is properly cleaned up.

defer func() {
	if err != nil {
		metricsTracerFactory.Shutdown()
	}
}()

References

1. When initializing resources that require cleanup (such as metrics) inside a constructor, use named return errors and a defer block to clean up the resources if the constructor fails.

[gemini-code-assist]

Hardcoding log.Default() in production library code makes it difficult for library users to suppress, redirect, or customize background configuration polling logs (which can be noisy during transient network issues). Consider making the logger configurable via ClientConfig or using a quieter/discard logger by default if the user does not specify one.

[gemini-code-assist]

To prevent a potential nil pointer dereference panic if Close() is called on an uninitialized or partially initialized SessionClient (where mPool is nil), add a nil check before calling c.mPool.Close(), similar to the guards used for configManager and metricsTracerFactory.

if c.mPool != nil {
	c.closeErr = c.mPool.Close()
}

[gemini-code-assist]

In TestSessionClient_Close_Idempotent, discarding the cleanup function returned by newTestSessionClient leaks the in-memory bttest.Server because srv.Close() is never called. Since Close() is idempotent, you can safely defer cleanup() at the start of the test to ensure the server is properly shut down, while still manually testing multiple Close() calls on the client.

Suggested change

	sc, _ := newTestSessionClient(t) // don't defer cleanup — we drive Close manually below
	if err := sc.Close(); err != nil {
	t.Errorf("first Close: %v", err)
	}
	// Second Close must not panic and must not error.
	if err := sc.Close(); err != nil {
	t.Errorf("second Close: %v", err)
	}
	sc, cleanup := newTestSessionClient(t)
	defer cleanup()
	if err := sc.Close(); err != nil {
	t.Errorf("first Close: %v", err)
	}
	if err := sc.Close(); err != nil {
	t.Errorf("second Close: %v", err)
	}