Summary:
metric_entity.h is included by metrics.h which is included by a lot of other files.
Including boost/regex.hpp takes ~300ms and is almost never needed elsewhere.

Compile time of transaction_participant.cc (avg 10 runs):
- before changes: 15.377s
- after changes: 15.099s
Jira: DB-15082

Test Plan: Jenkins

Reviewers: yyan, asrivastava

Reviewed By: asrivastava

Subscribers: ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D41505