Instrument course email for DataDog

[wedaly]

Reviewer: @sarina

[sarina]

This looks right to me.

I'm trying to think - was there anything else we wanted to measure?

I think there's probably a couple other things we might want to know when this is live for courses (I'm thinking particularly, how many unique courses are sending mail, and maybe what the option that they're using is).

Talking with Miki I do plan to try to do some testing with 2-3 courses sending mail simultaneously, so being able to somehow distinguish or know about multiple courses performing mail simultaneously might be a good idea.

[sarina]

Just fyi it's not strictly necessary to run this check. In Python,

tag[:INDEX]

will slice the string from index 0 up to but not including INDEX, even if the string is shorter:

>>> tag = 'hello'
>>> tag[:200]
'hello'

[wedaly]

I learn something new everyday :) Updated to use slices to enforce string length limit.

[sarina]

👍