feat(metrics): add public constructors for metric data types

[darklight3it]

Fixes #3469

Changes

Added public constructors to metric data types, allowing users to use the public PushMetricExporter::export.

Rationale:

• Types with one or more optional fields use the builder pattern, where the builder explicitly lists mandatory fields. This matches the established pattern in opentelemetry-sdk, opentelemetry-otlp, and other crates.
• Types with only required fields use a new method.

Summary:

Type	Constructor	Required fields	Optional (with_*) fields
ResourceMetrics	builder()	—	resource, scope_metrics
ScopeMetrics	builder()	—	scope, metrics
Metric	builder(name, data)	name, data	description, unit
Gauge<T>	builder(data_points, time)	data_points, time	start_time
GaugeDataPoint<T>	builder(value)	value	attributes, exemplars
Sum<T>	new(...)	data_points, temporality, is_monotonic, start_time, time	—
SumDataPoint<T>	builder(value)	value	attributes, exemplars
Histogram<T>	new(...)	data_points, temporality, start_time, time	—
HistogramDataPoint<T>	builder(count, sum, bounds, bucket_counts)	count, sum, bounds, bucket_counts	attributes, min, max, exemplars
ExponentialHistogram<T>	new(...)	data_points, temporality, start_time, time	—
ExponentialHistogramDataPoint<T>	builder(count, sum, scale, zero_count, positive_bucket, negative_bucket)	count, sum, scale, zero_count, positive_bucket, negative_bucket	attributes, min, max, zero_threshold, exemplars
ExponentialBucket	new(offset, counts)	offset, counts	—
Exemplar<T>	builder(value, time)	value, time	filtered_attributes, span_id, trace_id

Example usage:

use opentelemetry::{InstrumentationScope, KeyValue};
use opentelemetry::time::now;
use opentelemetry_sdk::metrics::data::*;
use opentelemetry_sdk::metrics::{Temporality, PushMetricExporter};
use opentelemetry_sdk::Resource;

let gauge_dp = GaugeDataPoint::builder(42.0_f64)
    .with_attributes(vec![KeyValue::new("host", "localhost")])
    .build();

let gauge = Gauge::builder(vec![gauge_dp], now()).build();

let metric = Metric::builder("cpu.usage", AggregatedMetrics::F64(MetricData::Gauge(gauge)))
    .with_description("CPU usage percentage")
    .with_unit("%")
    .build();

let scope_metrics = ScopeMetrics::builder()
    .with_scope(InstrumentationScope::builder("my-library").build())
    .with_metrics(vec![metric])
    .build();

let resource_metrics = ResourceMetrics::builder()
    .with_resource(Resource::builder().with_service_name("my-service").build())
    .with_scope_metrics(vec![scope_metrics])
    .build();

exporter.export(&resource_metrics).await?;

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)

[linux-foundation-easycla]

• ❌ - login: @darklight3it / name: Davide Melfi . The commit (afdf3b5, 849ecf3, d3e1646, e42bbdc, f4c0e4f) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[codecov]

Codecov Report

❌ Patch coverage is 99.08884% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.4%. Comparing base (cef3317) to head (e42bbdc).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/data/mod.rs	99.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3489     +/-   ##
=======================================
+ Coverage   83.7%   84.4%   +0.7%     
=======================================
  Files        126     126             
  Lines      25386   25825    +439     
=======================================
+ Hits       21255   21818    +563     
+ Misses      4131    4007    -124     

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