metricstarttimeprocessor: avoid copy for gauges when adjusting metrics

[ridwanmsharif]

Component(s)

metricstarttimeprocessor

Is your feature request related to a problem? Please describe.

see #38594 (comment) and #38594 (comment) for more details.
We currently copy over and update the entire metric slice to make small edits. Can we do this in place better even though even though we are dropping some points.

Describe the solution you'd like

We could potentially adjust the metric in place and avoid the copy of the metrics slice.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners for processor/metricstarttime: @dashpole @ridwanmsharif. See Adding Labels via Comments if you do not have permissions to add labels yourself. For example, comment '/label priority:p2 -needs-triaged' to set the priority and remove the needs-triaged label.

[crobert-1]

Issue filed by code owner, removing needs triage