[feature] add prometheus metric filter

[fatelei]

i want to add a prometheus metric filter to go-restful

[emicklei]

great! but please do not add prometheus as a dependency to this package. Instead, create an example with its own go.mod file that uses your filter defined in that example.

[fatelei]

ok

[emicklei]

closing this now as you can send a PR for the example