feat(bigtable): enable new auth library and JWT for instance admin client - #20013

🤖 I have created a release *beep* *boop*
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##
[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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