go/oasis-node/cmd/common/metrics: Fix label escaping

[kostko]

No description provided.

[gw0]

It fixes the issue at hand, but maybe an allowlist approach would be better?

[codecov]

Codecov Report

Merging #3678 (61a803c) into master (60545db) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3678      +/-   ##
==========================================
- Coverage   66.76%   66.73%   -0.04%     
==========================================
  Files         393      393              
  Lines       38388    38388              
==========================================
- Hits        25631    25618      -13     
- Misses       9087     9090       +3     
- Partials     3670     3680      +10     

Impacted Files	Coverage Δ
go/oasis-node/cmd/common/metrics/metrics.go	18.11% <100.00%> (+0.78%)	⬆️
go/consensus/tendermint/apps/governance/query.go	83.33% <0.00%> (-16.67%)	⬇️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
go/worker/upgrade/worker.go	67.70% <0.00%> (-5.21%)	⬇️
go/storage/mkvs/tree.go	86.95% <0.00%> (-4.35%)	⬇️
go/consensus/tendermint/full/light.go	48.93% <0.00%> (-4.26%)	⬇️
go/consensus/tendermint/full/services.go	80.00% <0.00%> (-4.17%)	⬇️
go/worker/common/committee/node.go	70.61% <0.00%> (-3.61%)	⬇️
go/storage/api/root_cache.go	78.33% <0.00%> (-3.34%)	⬇️
.../consensus/tendermint/apps/roothash/state/state.go	71.25% <0.00%> (-2.50%)	⬇️
... and 24 more

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