Pass around contexts in as sentry intends

[GTB3NW]

• Any spans within the handler will now inherit the interceptor span
• If the sentry-trace metadata exists it will update the span so that it continues. This is the exact approach it takes with the http library except that does it with headers.

[GTB3NW]

I'm going to submit a couple of other changes and make it a slightly bigger PR, so please don't merge for now

[GTB3NW]

I think this should work. I'm going to run some local testing first and confirm it works as I intended (traces work between services). I've tried to keep it as idiomatic as the http library. I had to copy over the logic from sentry's lib because they don't expose it annoyingly (they do for the equivalent client calls). I'm hoping to upstream this method to read from grpc metadata since I imagine it's a useful function for others too.

[GTB3NW]

Ohh and I took the liberty of updating sentry-go

[GTB3NW]

I've tested, the span upgrade fails gracefully if the metadata isn't there. I've not had chance to test the client. I'll test that next week and post an update here :)