[skip ci] Add doctest for MetricsLambda

[sdesrozis]

Addresses #2265

Description: Add doctest for MetricsLambda

@ydcjeff

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

@sdesrozis seems like you have to put [skip ci] to the commit and not the PR title to skip the CI.
And bravo for sending 7 PRs on docstrings 💪

[sdesrozis]

@sdesrozis seems like you have to put [skip ci] to the commit and not the PR title to skip the CI.

Ah ah...

And bravo for sending 7 PRs on docstrings 💪

Yep, I think it's finished (still merge to do) for metrics.

Now let's do the same for param schedulers.