Consistent prefix for system events and metrics. Closes #425

[mthenw]

What did you implement:

Closes #425

How did you implement it:

All metrics and system events are not prefixed with eventgateway prefix. gateway was too ambiguous.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: YES

[codecov]

Codecov Report

Merging #486 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #486   +/-   ##
=======================================
  Coverage   70.77%   70.77%           
=======================================
  Files          37       37           
  Lines        2190     2190           
=======================================
  Hits         1550     1550           
  Misses        576      576           
  Partials       64       64

Impacted Files	Coverage Δ
httpapi/metrics.go	100% <ø> (ø)	⬆️
router/metrics.go	35.29% <ø> (ø)	⬆️
event/event.go	78.12% <0%> (ø)	⬆️

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[mthenw]

This coverage is somehow broken, not sure why.

[sebito91]

LGTM!

[sebito91]