metrics Gauge cleanup

[brianolson]

Summary

Gauge no longer has a timeout.

Gauge that didn't change used to disappear from results; this could make a non-zero value appear zero, confusing metrics.

gaugeCommon.go split was confusing, merge back into gauge.go

Test Plan

Refactor continue to pass existing tests.

[codecov-commenter]

Codecov Report

Merging #2508 (4c7e5f8) into master (abc4058) will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2508      +/-   ##
==========================================
+ Coverage   46.86%   46.88%   +0.02%     
==========================================
  Files         345      347       +2     
  Lines       55400    55519     +119     
==========================================
+ Hits        25961    26031      +70     
- Misses      26503    26545      +42     
- Partials     2936     2943       +7     

Impacted Files	Coverage Δ
util/metrics/gauge.go	68.00% <50.00%> (+4.78%)	⬆️
ledger/applications.go	56.25% <0.00%> (-10.92%)	⬇️
crypto/merkletrie/trie.go	66.42% <0.00%> (-2.19%)	⬇️
crypto/merkletrie/node.go	91.62% <0.00%> (-1.87%)	⬇️
data/transactions/logic/eval.go	90.51% <0.00%> (-1.50%)	⬇️
data/transactions/logic/assembler.go	80.83% <0.00%> (-0.93%)	⬇️
ledger/eval.go	76.38% <0.00%> (-0.30%)	⬇️
network/wsNetwork.go	60.73% <0.00%> (-0.29%)	⬇️
data/transactions/logic/doc.go	82.75% <0.00%> (ø)
data/transactions/logic/opcodes.go	100.00% <0.00%> (ø)
... and 13 more

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[winder]

Removing this would be a problem if someone decides to use the label feature. It looks like that allows for creating dynamic guages based on user-defined keys. We don't currently use the dynamic labels feature in the networking code. In theory, someone might add a gauge by session ID (i.e. ping per connection). At that point removing this function would become be issue since it would grow unbounded.

A couple of possible solutions:

1. Remove the labels feature (it seems to only be used by segment.go:EnterSegment and that function is not called anywhere).
2. Return from this function immediately when len(valueIndices) <= 1
3. Set maxMetricRetensionDuration to something very large in config.json

[brianolson]

MakeSegement EnterSegment LeaveSegment never occur in the code. It's completely unused and I'm tempted to delete segment.go entirely.
labels is also unused, I checked all uses of Gauge.Set() and labels is always nil.

Since there's no problem with the code as it is but only as it might be, I'd like to finish this bugfix and file a new issue for further util/metrics cleanup of dead code.

[winder]

Mostly good, I just spot one possible problem

[winder]

Approved pending a new ticket is created. In the new ticket could you mention that StringGauge has the same problem?