Adding metadata header to client requests

[devshorts]

This PR adds in basic client telemetry that for now only includes timing information. The telemetry is sent back as a sidecar payload in an HTTP header of:

X-Stripe-Client-Telemetry

With a JSON payload format of:

{ 
  "last_request_metrics": {
      "request_id": "...",
      "request_duration": 1.23
  }
}

By sending as a sidecar http header we avoid an extraneous network call at the cost of only being able to track the previous request.

Adding in this telemetry will allow stripe to help diagnose latency and other connection issues as well as provide a space to augment future telemetry data.

Telemetry needs to be opted into by setting:

Stripe.enable_telemetry = true

And can be disabled at any time by toggling that flag (the value is not cached)

[devshorts]

r? @ob-stripe @brandur-stripe

[brandur-stripe]

LGTM.

[brandur-stripe]

Released as 3.31.0.