middleware/metrics: survive restart

[miekg]

Keep the handler running during restart. Stopping and starting the
handler results in "address in use" - sometimes, meaning the reload
will be flaky. In turn this behavior means any changes to the monitor
stanza are not picked up.

[codecov-io]

Codecov Report

Merging #542 into master will not change coverage.
The diff coverage is 76.19%.

@@           Coverage Diff           @@
##           master     #542   +/-   ##
=======================================
  Coverage   39.31%   39.31%           
=======================================
  Files         121      121           
  Lines        6069     6069           
=======================================
  Hits         2386     2386           
  Misses       3497     3497           
  Partials      186      186

Impacted Files	Coverage Δ
middleware/metrics/setup.go	39.21% <ø> (ø)	✅
middleware/metrics/metrics.go	80% <84.21%> (ø)	✅

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