Allow more fine grained disabling of http module features

[watson]

It's not possible to switch off the automatic creation of HTTP request transactions when an incoming HTTP request is detected without also switching off tracing of outgoing HTTP requests. This is because they are both controlled by this cone setting: disableInstrumentations: ['http'].

It would be very useful - both to users, but especially also during debugging - if it was possible to control the two features separately.

Considerations

Today there's a one to one relationship between npm package names and the content of the disableInstrumentations config option. I don't think it would be a good idea to introduce an exception just to solve this case.

As an alternative we could consider if the config option instead should focus on span types, so instead of disabling pg, you would disable db.postgresql (the full type is db.postgresql.query, but I guess we can just match against the start of the string, which also means that we could write db to disable all db spans).

There's pros and cons to this, so it should be considered thoroughly.

[watson]

Suggested solution: disableInstrumentations: ['http'] should only disable instrumentation of http.request. Automatic creation of transactions based on incoming http requests should be controlled by another config option, e.g. monitorIncomingHTTPRequests which defaults to true.

[watson]

If this turns out to be complicated, we can move it to 3.0.0.