v1.29.0 Release

Bigtable:

BREAKING CHANGES:

• Experimental functions matching Async* in bigtable::TableAdmin and
  bigtable::InstanceAdmin have been removed from the public API. See
  #5923 for more details.

Other changes:

• feat(bigtable): add Backup IAM Policy snippets (#6847)
• doc(bigtable): revamp examples README (#6839)
• docs: fix connection pool size for samples (#6834)
• cleanup(doc): add IAM examples to Bigtable gRPC doc, fix typos (#6838)
• cleanup(bigtable)!: remove admin async methods from public API (#6711)

Pub/Sub:

• fix: cast to std::string which works with string and Cord (#6850)
• doc(pubsub): an example for publisher flow control (#6784)
• feat(pubsub): flow control for Publisher (#6678)
• feat(pubsub): use unary RPCs for Ack/Nack (#6674)
• feat(pubsub): direct Ack/Nack through SubscriberStub (#6666)

Storage:

• fix(storage): allow overwrites in parallel uploads (#6908)
• feat(GCS+gRPC): capture streaming RPCs metadata (#6902)
• fix(storage): use correct header for XML pre-conditions (#6903)
• fix(storage): use JSON for reads with pre-conditions (#6900)
• fix(storage): treat 304 errors as kFailedPrecondition (#6897)
• feat(storage): micro-optimization for downloads (#6892)
• fix(GCS+gRPC): avoid duplicate headers in downloads (#6891)
• feat(storage): implement "public access prevention" (#6755)
• fix(GCS+gRPC): resumable uploads can resume (#6881)
• feat(storage): share client in throughput benchmark (#6882)
• feat(storage): make auto-finalization optional (#6874)
• fix(storage): alternative endpoints and Host header (#6864)
• fix(storage): crashes with tracing enabled (#6870)

Spanner:

• feature(spanner): low-cost instances (#6895)
• refactor(spanner): Use background threads for admin LROs (#6853, #6835)
• feature(spanner): add optimizer_statistics_package to QueryOptions (#6727)
• fix(spanner): pick up tracing-option defaults in SetBasicDefaults() (#6691)

Common Libraries:

• feat(common): support LROs (long-running operations) use background threads
  (#6855, #6831, #6824, #6823, #6822, #6820, #6816, #6804)
• fix(common): remove blocking call in MinimalIamCredentials (#6813)