Pt mongodb stalk

[palazuelosagustin]

• The contributed code is licensed under GPL v2.0
• Contributor Licence Agreement (CLA) is signed
• util/update-modules has been ran
• Documentation updated
• Test suite update

[it-percona-cla]

All committers have signed the CLA.

[svetasmirnova]

Thank you for the contribution. Please follow the rules:

• Documentation for Bash tools should be placed inline. Check other tools like pt-mysql-summary or pt-stalk. There should not be a separate README file for Bash tools.
• Sign contributor agreement
• Create a regression test for your changes, see examples in the t/TOOLNAME directories

Please also consider if this change fits existing pt-stalk, as a new option.

[palazuelosagustin]

Changes made:

  - Removed bin/pt-mongodb-stalk.README.
  - Moved the useful README content into inline POD inside bin/pt-mongodb-stalk.
  - Added regression test: t/pt-mongodb-stalk/pt-mongodb-stalk.t.
  - Test uses fake MongoDB/OS commands, so it does not require a real MongoDB instance.
  - Test covers --no-stalk, expected output files, currentOp allUsers:true, and retained non-empty mongotop.err.
  
  On the pt-stalk question: I’d argue this is better as a separate tool for now. pt-stalk is tightly MySQL-specific around mysql, mysqladmin, SHOW GLOBAL STATUS, SHOW PROCESSLIST, and MySQL option passthrough.
  Folding MongoDB in would require a larger abstraction of connection handling, triggers, and collectors rather than a simple option.