6195: Creation of DelegatingMetricData class

[vasantteja]

This Change

Context

This PR tries to solve this ticket. I preserved the original PR that was initially raised to add this class but I added a couple other enhancements that were mentioned in the PR. @jack-berg proposed two approaches for exposing other PointData interfaces:

1. Creating Delegating Classes
2. Adding Static Create Methods to Interfaces

I went with the second approach because it keeps the API simpler, consistent and avoids code duplication..

Implementation Example

Finally I am thinking people might be using this class with Metric Exporter as below:

import io.opentelemetry.sdk.common.CompletableResultCode;
import io.opentelemetry.sdk.common.InstrumentationScopeInfo;
import io.opentelemetry.sdk.metrics.data.Data;
import io.opentelemetry.sdk.metrics.data.DelegatingMetricData;
import io.opentelemetry.sdk.metrics.data.MetricData;
import io.opentelemetry.sdk.metrics.data.MetricDataType;
import io.opentelemetry.sdk.metrics.export.MetricExporter;
import io.opentelemetry.sdk.resources.Resource;

import java.util.ArrayList;
import java.util.Collection;
import java.util.function.Function;

/**
 * A MetricExporter that transforms metrics using a custom DelegatingMetricData implementation
 * before exporting them through a delegate exporter.
 */
public class TransformingMetricExporter implements MetricExporter {
    private final MetricExporter delegate;
    private final Function<MetricData, MetricData> transformer;

    /**
     * Creates a new TransformingMetricExporter.
     *
     * @param delegate The underlying exporter to send metrics to
     * @param namePrefix Prefix to add to all metric names
     * @param scaleFactor Factor to multiply all numeric values by (if applicable)
     */
    public TransformingMetricExporter(MetricExporter delegate, String namePrefix, double scaleFactor) {
        this.delegate = delegate;
        this.transformer = metric -> new CustomMetricData(metric, namePrefix, scaleFactor);
    }

    @Override
    public CompletableResultCode export(Collection<MetricData> metrics) {
        // Transform each metric before delegating to the wrapped exporter
        Collection<MetricData> transformedMetrics = new ArrayList<>(metrics.size());
        for (MetricData metric : metrics) {
            transformedMetrics.add(transformer.apply(metric));
        }
        return delegate.export(transformedMetrics);
    }

    @Override
    public CompletableResultCode flush() {
        return delegate.flush();
    }

    @Override
    public CompletableResultCode shutdown() {
        return delegate.shutdown();
    }

    /**
     * Custom implementation of DelegatingMetricData that modifies metric names and scales values.
     */
    private static class CustomMetricData extends DelegatingMetricData {
        private final String name;
        private final double scaleFactor;

        CustomMetricData(MetricData delegate, String namePrefix, double scaleFactor) {
            super(delegate);
            this.name = namePrefix + delegate.getName();
            this.scaleFactor = scaleFactor;
        }

        @Override
        public String getName() {
            return name;
        }

        @Override
        public Data<?> getData() {
            // Here you could implement value scaling logic based on the metric type
            // For example, you could create custom scaled versions of:
            // - LongSumData/DoubleSumData values
            // - HistogramData/ExponentialHistogramData values
            // - etc.
            
            return super.getData();
        }
        
        @Override
        public String toString() {
            return "CustomMetricData{name='" + name + "', scaled by " + scaleFactor + 
                   ", delegate=" + super.toString() + "}";
        }
    }
}

[codecov]

Codecov Report

Attention: Patch coverage is 73.61111% with 19 lines in your changes missing coverage. Please review.

Project coverage is 89.81%. Comparing base (3796992) to head (97a8cec).
Report is 37 commits behind head on main.

Files with missing lines	Patch %	Lines
...lemetry/sdk/metrics/data/DelegatingMetricData.java	95.65%	0 Missing and 2 partials ⚠️
...pentelemetry/sdk/metrics/data/DoublePointData.java	0.00%	2 Missing ⚠️
...telemetry/sdk/metrics/data/HistogramPointData.java	0.00%	2 Missing ⚠️
.../opentelemetry/sdk/metrics/data/LongPointData.java	0.00%	2 Missing ⚠️
...telemetry/sdk/metrics/data/DoubleExemplarData.java	0.00%	1 Missing ⚠️
.../sdk/metrics/data/ExponentialHistogramBuckets.java	0.00%	1 Missing ⚠️
...try/sdk/metrics/data/ExponentialHistogramData.java	0.00%	1 Missing ⚠️
...dk/metrics/data/ExponentialHistogramPointData.java	0.00%	1 Missing ⚠️
...a/io/opentelemetry/sdk/metrics/data/GaugeData.java	0.00%	1 Missing ⚠️
.../opentelemetry/sdk/metrics/data/HistogramData.java	0.00%	1 Missing ⚠️
... and 5 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7159      +/-   ##
============================================
- Coverage     89.85%   89.81%   -0.05%     
- Complexity     6613     6643      +30     
============================================
  Files           740      756      +16     
  Lines         19991    20079      +88     
  Branches       1966     1976      +10     
============================================
+ Hits          17963    18033      +70     
- Misses         1439     1455      +16     
- Partials        589      591       +2     

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