It would be nice to support multiple Prometheus registries

[davidhopkins]

At the moment because the default registry is used this is not possible. For example if you have an application with two Gin servers the second gin-prometheus handler will fail with an error like:

time="2021-03-09T17:43:51Z" level=error msg="requests_total could not be registered in Prometheus" error="duplicate metrics collector registration attempted"
time="2021-03-09T17:43:51Z" level=error msg="request_duration_seconds could not be registered in Prometheus" error="duplicate metrics collector registration attempted"
time="2021-03-09T17:43:51Z" level=error msg="response_size_bytes could not be registered in Prometheus" error="duplicate metrics collector registration attempted"
time="2021-03-09T17:43:51Z" level=error msg="request_size_bytes could not be registered in Prometheus" error="duplicate metrics collector registration attempted"

Cheers!

[codysk]

#50
I have same issue and made a pr for it.
Maybe it can help you :)