[AlvaroStream]

Descriptions of the changes in this PR:

Fix #4917

Motivation

The BKDLConfig stored in a DistributedLog namespace carries the ZooKeeper servers that the BookKeeper client and the
reader ZooKeeper client connect to. When these are in-cluster service names that a client outside the cluster cannot
resolve, while the namespace URI holds a reachable address, the client has no way to prefer the URI's servers.
See #4917 for details.

Changes

• Add DistributedLogConfiguration#zkServersIgnorePattern (getZkServersIgnorePattern / setZkServersIgnorePattern),
  default empty so behavior is unchanged.
• Add BKNamespaceDriver#selectZkServers: if a stored ZooKeeper servers value matches the pattern, use the
  namespace URI's servers instead and log a warning. Applied to the DL writer/reader and BK writer/reader values, and
  to the reader servers in ZKMetadataAccessor.
• Add TestBKNamespaceDriverZkServers: unit tests for the selection and configuration, a baseline read/write, and an
  end-to-end read/write against a namespace whose stored BookKeeper ZooKeeper servers are an unresolvable
  *.svc.cluster.local name with the pattern set. The same end-to-end test fails when the pattern is not set.

What is affected

• Code: only stream/distributedlog/core. Anything that opens a DistributedLog namespace through
  BKNamespaceDriver or ZKMetadataAccessor can use the new setting: client applications, and the modules in this
  repo that use the library (stream/server, tools/perf, stream/distributedlog/io/dlfs).
• New public interface: the client configuration key zkServersIgnorePattern. It defaults to empty, so behavior
  is unchanged for everyone who does not set it.
• Not affected: ZooKeeper and bookies, bookkeeper-server, the stored namespace config (nothing is written and
  there is no metadata or wire format change), and clients with and without the setting can use the same namespace.
• Not covered: the admin tools DistributedLogTool and DLAuditor still read the stored ZooKeeper servers
  directly and ignore the new setting.

Testing

mvn -pl stream/distributedlog/core -am test -Dtest='TestBKNamespaceDriverZkServers,TestDistributedLogConfiguration,TestZkMetadataResolver'
(20 tests pass, checkstyle passes).

[AlvaroStream]

Closed, I tried to reproduce the issue locally and it was not possible.

This won't resolve the problem.