Update to support prom-client 13.x

[robross0606]

This plugin only supports precisely prom-client 12.0.0. It won't work with 11.x or the newer 13.x. It makes it quite difficult to use multiple plugins when they each have such specific and mutually exclusive prom-client requirements.

[tomasAlabes]

Just in case someone is interested, I added support for this in my fork: https://github.com/tomasAlabes/apollo-prometheus-plugin