Feature: mongo-go-driver instrumentation

[axw]

Add tracing (and metrics?) for https://github.com/mongodb/mongo-go-driver

[axw]

We can trace requests by implementing a https://godoc.org/github.com/mongodb/mongo-go-driver/event#CommandMonitor. The events are stateless, so we'll need to maintain a mapping of conn+request ID to span.

[axw]

To extract details from MongoDB errors, we should implement #447.

[axw]

I've opened https://jira.mongodb.org/browse/GODRIVER-837, proposing an extension to the driver to enable implementations of the monitoring API to store arbitrary data with a request. Until/unless that is an option, we'll have to maintain our own map of active requests.