support custom prometheus registry

[jack-evans]

Steps for Reproduction

Currently, the prometheus metrics implementation for the cache only supports the use of the default registerer. This introduces global state into peoples applications and makes it difficult for people with custom registerers to use the prometheus metrics implementation.

Expected behavior:

allow people the option of using a custom metrics registry

Actual behavior:

No metrics for people using a custom prometheus metrics registry

Platforms:

Include browser, operating system and respective versions

Versions:

Which versions are you running?