v1.25.0 Release

Bigtable:

• We have marked the asynchronous versions of the administrative functions as
  deprecated. These functions were experimental and we do not think they add
  value to our customers. Removing them would simplify the code and free
  development time for further features and cleanups. More information in
  #5923.

• The legacy IAM functions were marked as deprecated via doxygen comments.
  Now they should generate warnings at the call site, depending on your
  compiler settings. See #5929 for more information.

• feat(bigtable): add CMEK attributes to admin APIs (#5921)

• feat(bigtable): limit default connection pool size (#5881)

• doc: workarounds for Bazel and path length problems (#5869)

• doc(bigtable): add configure_connection_pool sample (#5839)

Pub/Sub:

• Implemented support for "schemas". This feature allows you to define the
  schema of the messages accepted by a Topic. The schemas can be defined using
  AVRO or Protocol Buffers. This is a public preview feature of Cloud Pub/Sub,
  the APIs are found in the ::google::cloud::pubsub_experimental namespace.

Spanner:

• feat(spanner): Point-In-Time Recovery (PITR) (#5906)
  This is a major new feature in Spanner, supporting backups and restores at
  a given timestamp.
• refactor(spanner): spanner::Timestamp/protobuf::Timestamp conversions… (#5876)
• feat(spanner): statistics returned for a committed transaction (#5809)

Storage:

• The legacy IAM functions were marked as deprecated via doxygen comments.
  Now they should generate warnings at the call site, depending on your
  compiler settings. See #5929 for more information.

• doc(storage): Update UBLA documentation to reflect its current status (#5870)
  It has been GA for a long time, but was still described as not in our
  comments.

• fix(storage): remove unneeded dependency (#5798)
  The backwards compatibility target and package (storage_client) required
  the googleapis protos.