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.
* Add simple instrumentation callback
We used to insert Faraday::Request::Instrumentation into our Faraday middleware
stack to be able to instrument Stripe calls with StatsD. With Faraday being
removed in version 5, this required some rework. This commit implements a simple
callback system that can be used with any kind of instrumentation system.
* Add a topic to Stripe::Instrumentation notifications
... and a :request topic to subscribe to
* Use a RequestEvent value object instead of positional args in callback
This way the RequestLogContext object doesn't get exposed externally. Since the
same value object can be received by multiple subscribers it is frozen to
prevent accidental mutations across threads.
* Relocate tests for instrumentation and add more tests
