Add Mongodb test example

[guel-codes]

This is a resubmission/continuation of #2882.

Additions:

• examples/mongodb/README.md
• examples/mongodb/locustfile.py
• locust/contrib/mongodb.py

Updates:

• Updated the event request type and name:
  • addressing -> https://github.com/locustio/locust/pull/2882#discussion_r1739637421
• Fixed MongoDBClient:
  • addressing -> https://github.com/locustio/locust/pull/2882#discussion_r1739639080

[cyberw]

Ignore the build error, its a github thing and I have fixed it in master...

[cyberw]

thx!