add span hooks for web framework integrations

[rochdev]

This PR adds span hooks for web framework integrations. The hooks allow users to make modifications to the span just before it's finished.

A few use cases:

• Changing the resource name to customize how traces are grouped in the UI
• Adding additional metadata

[rochdev]

@brettlangdon The current behavior for spans that are augmented is to keep both hooks. For example, an Express request will start its life as an http.request from the http integration, and then be converted to a express.request in the express integration. In such a case, hooks for both http and express will run before the request span is finished.

Do you think this behavior makes sense or should we simply keep the last hook, in this case the express one?

[brettlangdon]

For Python we are only going to implement integration specific hooks, e.g. express.request.

I assume for most people that is probably where they will implement hooks, but it seems harmless to support both?

[brettlangdon]

lgtm, implementation makes sense 👍

[rochdev]

I assume for most people that is probably where they will implement hooks, but it seems harmless to support both?

It's definitely an edge case. We could always reevaluate this if it ever causes an issue.