feat(bigtable): add Session primitives (AttemptOutcome, vRPC ctx, msgtype) - #20116

…ine (#20185)

## Summary

Builds on the vRPC primitives merged in #20116 by adding the two
building blocks that consumers need to actually run interceptor
pipelines with retries:

- **`ChainInterceptors`** (`internal/transport/chain.go`) — composes
multiple `Interceptor`s around a base `Handler`. First interceptor in
the list is outermost, so it sees setup before and teardown after the
rest.
- **`RetryingVRpc`** (`internal/transport/retrying.go`) — an
`Interceptor` that retries failed virtual RPCs with exponential backoff.
Default classification is Java-parity:
  - `StateUncommitted` → always retry (server never saw the frame).
- `StateTransportFailure` → retry only when `Idempotent` is true (server
may have applied).
- `StateServerResult` → retry only if the server attached
`errdetails.RetryInfo`; a bare server-explicit error (even `Unavailable`
/ `Aborted` / `DeadlineExceeded`) does NOT retry without explicit server
go-ahead.
- Callers with bespoke policies can set `ShouldRetry` to override the
default entirely.

Per-attempt tracing goes through `VRpcTracer`; the previous attempt's
error is tagged onto the next attempt's context via `WithPrevAttemptErr`
for downstream metrics/logging.

## Test plan

- [x] `go build ./internal/transport/...`
- [x] `go vet ./internal/transport/...`
- [x] `go test ./internal/transport/... -run
'Retrying|Chained|Interceptor' -count=1 -race -timeout=90s` — 12 new
tests pass:
- `TestChainedInterceptors_Success` — interceptor call ordering
(outer→inner→base→inner→outer)
- `TestRetryingVRpc_SuccessOnRetry` — recovers on attempt 3, tracer sees
all 3 starts/completes
- `TestRetryingVRpc_NonRetryableError` — `InvalidArgument` aborts after
1 attempt
- `TestRetryingVRpc_HonorServerRetryDelay` — server `RetryInfo`
overrides client backoff
  - `TestRetryingVRpc_MaxAttemptsExceeded` — stops at `MaxAttempts`
  - `TestRetryingVRpc_NonGrpcError` — raw Go errors are non-retryable
- `TestRetryingVRpc_UncommittedAlwaysRetries` — `StateUncommitted`
retries even when `Idempotent=false`
- `TestRetryingVRpc_TransportFailureIdempotent` —
`StateTransportFailure` retries with `Idempotent=true`
- `TestRetryingVRpc_TransportFailureNonIdempotentNoRetry` — no retry
when `Idempotent=false`
- `TestRetryingVRpc_ServerResultNotRetriedByDefault` — bare
`ServerResult` never retries (5 codes)
- `TestRetryingVRpc_ServerDeadlineExceededNoRetryByDefault` — server
`DEADLINE_EXCEEDED` alone is not retried
- `TestRetryingVRpc_ServerDeadlineExceededRetriesWithRetryInfo` —
`DEADLINE_EXCEEDED` + `RetryInfo` is retried
- [x] `goimports -d` clean
- [x] `golint` clean

🤖 I have created a release *beep* *boop*
---


##
[1.51.0](bigtable/v1.50.0...bigtable/v1.51.0)
(2026-07-23)


### Features

* **bigtable:** Add ChainInterceptors and RetryingVRpc for vRPC pipeline
([#20185](#20185))
([c7a832a](c7a832a))
* **bigtable:** Add ClientConfigurationManager
([#19986](#19986))
([3a8f927](3a8f927))
* **bigtable:** Add debug tag counter (recordDebugTag / assertDebugTag)
([#20114](#20114))
([3c97590](3c97590))
* **bigtable:** Add lazyPool helper for on-demand session pool opening
([#20182](#20182))
([f6ae3fb](f6ae3fb))
* **bigtable:** Add PeakEwma continuous time-decay latency tracker
([#20187](#20187))
([9d124ef](9d124ef))
* **bigtable:** Add PoolSizer for server-driven session pool capacity
([#20189](#20189))
([57ebbeb](57ebbeb))
* **bigtable:** Add session package with SessionClient + SessionTableAPI
interfaces
([#20180](#20180))
([4b82fd2](4b82fd2))
* **bigtable:** Add Session primitives (AttemptOutcome, vRPC ctx,
msgtype)
([#20116](#20116))
([e1011e2](e1011e2))
* **bigtable:** Add Session state enum
([#19981](#19981))
([0748972](0748972))
* **bigtable:** Add SessionThrottler / AdaptiveSessionThrottler for
OpenSession pacing
([#20184](#20184))
([02e3c6d](02e3c6d))
* **bigtable:** Add SessionThrottler / AdaptiveSessionThrottler for
OpenSession pacing
([#20184](#20184))
([29be83e](29be83e))
* **bigtable:** Add sessionTracer for per-Session lifecycle + vRPC
metrics
([#20190](#20190))
([a466345](a466345))
* **bigtable:** Enable new auth library and JWT for instance admin
client
([#20013](#20013))
([21c4a44](21c4a44))
* **bigtable:** Modularize channel priming behind a ChannelPrimer
interface
([#20027](#20027))
([5214ab7](5214ab7))
* **bigtable:** Modularize Direct Access compatibility check
([#19987](#19987))
([a25e93d](a25e93d))
* **o11y:** Regenerate clients for LRO tracing
([#20107](#20107))
([779074e](779074e))


### Bug Fixes

* **bigtable:** Default cluster/zone in toOtelMetricAttrs to avoid
Monitoring reject
([#20178](#20178))
([14493f4](14493f4))
* **bigtable:** Eliminate stats-handler MD race in internal/metrics
tracer
([#20158](#20158))
([c387066](c387066))

---
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Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>