Report a per-http call metric

[ryanpq]

Checklist
I agree to the terms within the OpenFGA Code of Conduct.
Describe the problem you'd like to have solved
Right now, the metrics we emit are per-SDK method call at the API level (e.g. BatchCheck is multiple metrics for each underlying Check).

Users want to keep track of each underlying http call

Describe the ideal solution
We should expose another metric fga-client.http_request.duration that is emitted per-http request sent by the SDK

Alternatives and current workarounds
No response

References
No response

Additional context
Part of openfga/sdk-generator#418