[native] Add a metric to track expected reduction of memory and minor fixes

[bikramSingh91]

Summary:

• Add a metric presto_cpp.memory_pushback_expected_reduction_bytes
  to track expected reduction in memory after a pushback attempt as
  this can be different from actual reduction due to other threads
  allocating simultaneously
• Fix registeration of a pushback metric
• Make periodicCb() API non-const to allow it to update internal
  state if required

[steveburnett]

Consider adding doc to properties.rst or properties-session.rst as appropriate.

[bikramSingh91]

Consider adding doc to properties.rst or properties-session.rst as appropriate.

@steveburnett Thank you for your review.
This only adds a metric but no properties. Is there a doc that keeps track of metrics instead that I need to update?

[tanjialiang]

LGTM, thanks

[jaystarshot]

Please consider adding release notes following our release notes guide - link.

Using below for now

Add a metric presto_cpp.memory_pushback_expected_reduction_bytes
to track expected reduction in memory after a pushback attempt. :pr:`23872` 

[steveburnett]

Consider adding doc to properties.rst or properties-session.rst as appropriate.

@steveburnett Thank you for your review. This only adds a metric but no properties. Is there a doc that keeps track of metrics instead that I need to update?

Not that I am aware of.