Problem
=======

When a metadata lock is taken for a TRANSACTION duration,
and later modified with an EXPLICIT duration,
which for example happens with RENAME TABLE,
the table performance_schema.metadata_locks
did not show the correct lock DURATION.

Root cause
==========

The code in sql/mdl.cc is not instrumented to update
the duration in the performance schema instrumentation,
and beside, the instrumentation does not provide an api
to do so.

Fix
===

For components:
Add a set_metadata_lock_duration() instrumentation api
in the psi_mdl service, and increment the service version to psi_mdl_v2.

For plugins or server code:
Add a set_metadata_lock_duration() instrumentation api
in struct PSI_mdl_service, and increment version to PSI_mdl_service_v2.

Implement a mysql_mdl_set_duration() helper.

Call mysql_mdl_set_duration() from MDL_context::set_lock_duration().

Supporting all the combinations for services / plugins,
and for versions 1 and 2 is not strictly necessary,
given that there is only one known caller
of the metadata lock instrumentation (in sql/mdl.cc),
this is done to have a reference implementation using versions
in the code base.
Backward compatibility for any callers is provided.

Approved by: Chris Powers <chris.powers@oracle.com>