Fix telemetry header

[ob-stripe]

r? @remi-stripe
cc @stripe/api-libraries

I just noticed a bug with the telemetry header: it's serialized as {"last_request_metrics":{"request_id":["req_123"],"request_duration_ms":234}}, i.e. request_id is an array instead of a string.

This is because the response headers returned by the requestor is a map of string to list of strings (because a header may be repeated and have multiple values).

In this case we're fairly confident that there will only be a single value for the Request-Id header, so we fix this by fetching the first value.

[remi-stripe]

Approving but I don't understand why the test didn't change

[ob-stripe]

Approving but I don't understand why the test didn't change

We're stubbing the request method in the test, and we were returning the incorrect type (i.e. a map of string->string instead of a a map of string->list of strings). This is now fixed.

[ob-stripe]

Released as 7.7.1.