go/worker/keymanager: Add key manager worker metrics

[peternose]

Currently the key manager worker lacks metrics that would enable the operator to gain some useful insights into its operation. We should add the following metrics:

• Number of key manager policy updates
• Number of compute runtimes using the key manager
• Number of remote EnclaveRPC requests via P2P (separate per method)

[codecov]

Codecov Report

Merging #4877 (e39dc3e) into master (d0bb246) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4877      +/-   ##
==========================================
- Coverage   66.54%   66.45%   -0.10%     
==========================================
  Files         452      454       +2     
  Lines       50418    50431      +13     
==========================================
- Hits        33551    33514      -37     
- Misses      12705    12743      +38     
- Partials     4162     4174      +12     

Impacted Files	Coverage Δ
go/worker/keymanager/init.go	61.53% <100.00%> (+0.60%)	⬆️
go/worker/keymanager/metrics.go	100.00% <100.00%> (ø)
go/worker/keymanager/p2p/metrics.go	100.00% <100.00%> (ø)
go/worker/keymanager/p2p/server.go	80.00% <100.00%> (+2.22%)	⬆️
go/worker/keymanager/worker.go	64.01% <100.00%> (+0.15%)	⬆️
.../worker/compute/executor/committee/transactions.go	81.81% <0.00%> (-6.82%)	⬇️
go/worker/common/p2p/dispatch.go	71.52% <0.00%> (-5.56%)	⬇️
go/runtime/host/multi/multi.go	70.58% <0.00%> (-3.68%)	⬇️
go/worker/common/committee/keymanager.go	88.33% <0.00%> (-3.34%)	⬇️
go/runtime/registry/host.go	69.38% <0.00%> (-2.86%)	⬇️
... and 13 more

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[kostko]

Nice, looks good!

One thing that you need to add is a changelog fragment that describes what changes are there when making a release.