Performance improvements for the metricstarttime processor (open-telemetry#40998)

#### Description

The existing implementation copied everything from the previous
resourcemetrics to a completely new resourcemetrics. However, the
component is marked as mutating data, so this PR just changes it to
mutate the batch of metrics in-place. This avoids a lot of copying.

From previous experience caching start times and values, storing
attributes and exemplars in a cache is very expensive unless you need
them. The second commit adds "minimal copy to" methods to avoid storing
attributes and exemplars.

#### Link to tracking issue
Fixes
open-telemetry#39400

#### Testing

Passes all existing unit tests.

Author: dashpole

processor/metricstarttimeprocessor/go.mod

@@ -3,6 +3,7 @@ module github.com/open-telemetry/opentelemetry-collector-contrib/processor/metri
 go 1.23.0
 
 require (
+	github.com/open-telemetry/opentelemetry-collector-contrib/pkg/pdatatest v0.129.0
 	github.com/open-telemetry/opentelemetry-collector-contrib/pkg/pdatautil v0.129.0
 	github.com/stretchr/testify v1.10.0
 	go.opentelemetry.io/collector/component v1.35.1-0.20250710172626-1ebb9b20cfa5
@@ -66,3 +67,7 @@ require (
 )
 
 replace github.com/open-telemetry/opentelemetry-collector-contrib/pkg/pdatautil => ../../pkg/pdatautil
+
+replace github.com/open-telemetry/opentelemetry-collector-contrib/pkg/pdatatest => ../../pkg/pdatatest
+
+replace github.com/open-telemetry/opentelemetry-collector-contrib/pkg/golden => ../../pkg/golden