Add eventKey to service discovery metric

[kcajmagic]

The metric would be wrong if the flag crossDatacenter was enabled.

[kcajmagic]

required for xmidt-org/scytale#109

[codecov]

Codecov Report

Merging #509 into main will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #509      +/-   ##
==========================================
+ Coverage   86.58%   86.62%   +0.03%     
==========================================
  Files         186      186              
  Lines        8321     8321              
==========================================
+ Hits         7205     7208       +3     
+ Misses        912      909       -3     
  Partials      204      204              

Impacted Files	Coverage Δ
service/metrics.go	100.00% <100.00%> (ø)
service/monitor/listener.go	81.42% <100.00%> (ø)
device/drain/drainer.go	96.96% <0.00%> (+1.51%)	⬆️

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[johnabass]

This looks good, but let Mike know this metric has another label. It may mess up his charts.