Adds a new instrumentation callback called `request_begin` which, as the
name suggests, is invoked before an HTTP request is dispatched. As
outlined originally in #900, the idea is that this will enable a set of
hooks that can be used for distributed tracing.
The PR also renames the existing `request` callback to `request_end`,
although the old name is still invoked for the time being for backwards
compatibility.
A special `user_data` property is passed to `request_begin` which allows
subscribers to set custom data that will be passed through to
`request_end` for any given request. This allows, for example, a user
assigned ID to be set for the request and recognized on both ends.
I chose the naming `_begin` and `_end` (as opposed to start/finish or
any other combination) based on the naming conventions of Ruby itself.
Fixes#900.
81407b6807