fix: gauge continuity

[fracasula]

Description

We must not reset the gauge value after an observation is taken, otherwise if no value is written before the next observation, we'll end up with a gap. Instead we should be showing the last value since it has not changed:

TheotelGauge.getValue() method is called inside the OTel RegisterCallback only where we have the following:

_, err = meter.RegisterCallback(func(ctx context.Context, o metric.Observer) error {
	if value := og.getValue(); value != nil {
		o.ObserveFloat64(g, cast.ToFloat64(value), og.attributes...)
	}
	return nil
}, g)

Notion Ticket

< Notion Link >

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.05 ⚠️

Comparison is base (458288a) 77.54% compared to head (209b9bb) 77.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   77.54%   77.50%   -0.05%     
==========================================
  Files          48       48              
  Lines        3135     3129       -6     
==========================================
- Hits         2431     2425       -6     
  Misses        581      581              
  Partials      123      123              

Impacted Files	Coverage Δ
stats/otel_measurement.go	73.80% <100.00%> (-3.28%)	⬇️

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