Sepatate Aggregate measure and collect functions

[fraillt]

Changes

Minor improvements in how aggregate functions for measure and collect is created.

• Makes it more convenient to create aggregate functions

-    pub(crate) fn last_value(&self) -> (impl Measure<T>, impl ComputeAggregation) {
-        let lv = Arc::new(LastValue::new(self.temporality, self.filter.clone()));
-        (lv.clone(), lv)
+    pub(crate) fn last_value(&self) -> AggregateFns<T> {
+        LastValue::new(self.temporality, self.filter.clone()).into()

This will be important for #2328, when we'll want to construct separate instances for different temporality

• Now there's only single Arc instance for measure and collect. Previously there was helper function that wrapped single Arc into another Arc for measure, and into Box for collect.

-    fn box_val<T>(
-        (m, ca): (impl internal::Measure<T>, impl internal::ComputeAggregation),
-    ) -> (
-        Arc<dyn internal::Measure<T>>,
-        Box<dyn internal::ComputeAggregation>,
-    ) {
-        (Arc::new(m), Box::new(ca))
-    }
-

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 96.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.9%. Comparing base (42b4f2f) to head (4813ad3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/pipeline.rs	78.5%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2492     +/-   ##
=======================================
- Coverage   77.9%   77.9%   -0.1%     
=======================================
  Files        123     123             
  Lines      22925   22888     -37     
=======================================
- Hits       17870   17839     -31     
+ Misses      5055    5049      -6     

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[utpilla]

This is a good simplification.