refactor: AggregatedMetrics as enum instead of dyn Aggregation

[fraillt]

Changes

• Introduced a new AggregatedMetrics enum, which store data reported by Instruments.
• Change Metric::data field from Box<dyn Aggregation> to AggregatedMetrics.
• ResourceMetrics is now Clone, because AggregatedMetrics is a simple enum that can be easily cloned.

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 71.90570% with 143 lines in your changes missing coverage. Please review.

Project coverage is 80.7%. Comparing base (f12833f) to head (e849cd7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...-sdk/src/metrics/internal/exponential_histogram.rs	55.8%	38 Missing ⚠️
opentelemetry-sdk/src/metrics/mod.rs	83.0%	31 Missing ⚠️
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	25 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	11 Missing ⚠️
opentelemetry-sdk/src/metrics/data/mod.rs	57.1%	9 Missing ⚠️
...pentelemetry-sdk/src/metrics/in_memory_exporter.rs	81.0%	7 Missing ⚠️
opentelemetry-sdk/src/metrics/internal/mod.rs	84.6%	6 Missing ⚠️
...pentelemetry-sdk/src/metrics/internal/aggregate.rs	76.1%	5 Missing ⚠️
...entelemetry-sdk/src/metrics/internal/last_value.rs	80.9%	4 Missing ⚠️
...pentelemetry-sdk/src/metrics/internal/histogram.rs	86.3%	3 Missing ⚠️
... and 2 more

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2857     +/-   ##
=======================================
- Coverage   80.9%   80.7%   -0.2%     
=======================================
  Files        124     124             
  Lines      23726   23703     -23     
=======================================
- Hits       19201   19146     -55     
- Misses      4525    4557     +32     

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[cijothomas]

See if you like this better:
main...cijothomas:opentelemetry-rust:cijothomas/new-metrics#diff-041ac5cb4c3201f64e52be34cdba07e769e2b3637d605ddf59852a66ef104762R7-R70

[fraillt]

From a user's perspective, it's more convenient to have a MetricData<T>, because this allows almost all code to be generic over metric data.

fn clone_inner<T: Clone>(data: &MetricData<T>) -> MetricData<T> {
    match data {
        MetricData::Gauge(gauge) => Gauge {
            data_points: gauge.data_points.clone(),
            start_time: gauge.start_time,
            time: gauge.time,
        }
        .into(),
        MetricData::Sum(sum) => Sum {
            data_points: sum.data_points.clone(),
            start_time: sum.start_time,
            time: sum.time,
            temporality: sum.temporality,
            is_monotonic: sum.is_monotonic,
        }
        .into(),
        MetricData::Histogram(histogram) => Histogram {
            data_points: histogram.data_points.clone(),
            start_time: histogram.start_time,
            time: histogram.time,
            temporality: histogram.temporality,
        }
        .into(),
        MetricData::ExponentialHistogram(exponential_histogram) => ExponentialHistogram {
            data_points: exponential_histogram.data_points.clone(),
            start_time: exponential_histogram.start_time,
            time: exponential_histogram.time,
            temporality: exponential_histogram.temporality,
        }
        .into(),
    }
}
match data {
    AggregatedMetrics::F64(metric_data) => AggregatedMetrics::F64(clone_inner(metric_data)),
    AggregatedMetrics::U64(metric_data) => AggregatedMetrics::U64(clone_inner(metric_data)),
    AggregatedMetrics::I64(metric_data) => AggregatedMetrics::I64(clone_inner(metric_data)),
}

[cijothomas]

LGTM. We can followup on the open comments (Moving resource out, removing i64 in aggregation etc.)