worker/metrics/sender: fix mutex copy bug in test
worker/metrics/sender: fix mutex copy bug in test

Fixes 1563628

Fixed accidental copy of a mutex in test.

Fixes LP 1563628 when #4938, #4937 and #4931 are comitted.

mjs/juju,dooferlad/juju,bac/juju,gabriel-samfira/juju,mjs/juju,axw/juju,waigani/juju,dimitern/juju,alesstimec/juju,frankban/juju,gabriel-samfira/juju,frankban/juju,reedobrien/juju,ericsnowcurrently/juju,bz2/juju,ericsnowcurrently/juju,gabriel-samfira/juju,mjs/juju,reedobrien/juju,reedobrien/juju,bogdanteleaga/juju,AdamIsrael/juju,gabriel-samfira/juju,howbazaar/juju,dimitern/juju,makyo/juju,kat-co/juju,axw/juju,bz2/juju,macgreagoir/juju,macgreagoir/juju,howbazaar/juju,axw/juju,anastasiamac/juju,dooferlad/juju,perrito666/juju,anastasiamac/juju,macgreagoir/juju,mikemccracken/juju,anastasiamac/juju,makyo/juju,AdamIsrael/juju,anastasiamac/juju,mikemccracken/juju,macgreagoir/juju,alesstimec/juju,anastasiamac/juju,gabriel-samfira/juju,dimitern/juju,mjs/juju,bac/juju,kat-co/juju,frankban/juju,bz2/juju,mikemccracken/juju,marcmolla/juju,macgreagoir/juju,kat-co/juju,bz2/juju,howbazaar/juju,ericsnowcurrently/juju,alesstimec/juju,perrito666/juju,howbazaar/juju,mikemccracken/juju,perrito666/juju,bac/juju,alesstimec/juju,marcmolla/juju,marcmolla/juju,axw/juju,reedobrien/juju,alesstimec/juju,AdamIsrael/juju,bac/juju,axw/juju,dimitern/juju,waigani/juju,perrito666/juju,kat-co/juju,mjs/juju,bogdanteleaga/juju,howbazaar/juju,bogdanteleaga/juju,mjs/juju,dooferlad/juju,bz2/juju,bac/juju,waigani/juju,ericsnowcurrently/juju,perrito666/juju,marcmolla/juju,makyo/juju,mikemccracken/juju,frankban/juju,ericsnowcurrently/juju,frankban/juju,dooferlad/juju,marcmolla/juju,mjs/juju,bogdanteleaga/juju,makyo/juju,dimitern/juju,AdamIsrael/juju,waigani/juju,reedobrien/juju,dooferlad/juju,kat-co/juju