Support Django DB Instrumentation

[zoidyzoidzoid]

Django 2.0 supports DB instrumentation which can be used to wrap DB queries and automatically trace them, which would be awesome, because it'd even support other services and SQL engines we don't necessarily support.

Honeycomb is using it here

[zoidyzoidzoid]

I can make a PR if this is something we'd like to support. There are some other useful fields in context['cursor'] that we might wanna add to traces too.

We probably need to better support Django < 2.0 which I haven't tested yet, and I still need to write tests, but I'd be keen to hear if my branch is on the right track or not.

[c24t]

@zoidbergwill if you're still interested in this it'd be a great addition to the django integration.

[zoidyzoidzoid]

@c24t I'd love to see this get shipped. I'll take a proper look this weekend or next week. I remember last time getting caught up working on some tests.