ObservableCounter should report absolute value

[pfdgithub]

What happened?

Steps to Reproduce

In api-metrics@0.29.2, there are synchronous api Counter and asynchronous api ObservableCounter.
But ObservableCounter does not report absolute value, this is not conforming to specification.

Note: Unlike Counter.Add() which takes the increment/delta value, the callback function reports the absolute value of the counter. To determine the reported rate the counter is changing, the difference between successive measurements is used.

According to my colleague, in the Java implementation, ObservableCounter report absolute value.
If that's true, this will cause cross-language developers to misunderstand the API.

Expected Result

The same value is reported each time.
1 - 1 - 1 - 1 - 1 - ...

Actual Result

The value is incremented with each report.
1 - 2 - 3 - 4 - 5 -...

Additional Details

refer:
#1564

OpenTelemetry Setup Code

import { metrics } from "@opentelemetry/api-metrics";

let counterInstrument;
let observableCounter = 0;

const meter = metrics.getMeter("nodejs-metrics");

counterInstrument = meter.createCounter("nodejs-counter");

meter
  .createObservableCounter("nodejs-observable-counter")
  .addCallback((observable) => {
    observable.observe(observableCounter);
  });

counterInstrument.add(1);

observableCounter = 1;

package.json

@opentelemetry/api-metrics@0.29.2

Relevant log output

// ---------- first report ----------

[
  {
    scope: { name: 'nodejs-metrics', version: '', schemaUrl: undefined },
    metrics: [
      {
        descriptor: {
          name: 'nodejs-counter',
          description: '',
          type: 'COUNTER',
          unit: '',
          valueType: 1
        },
        aggregationTemporality: 1,
        dataPointType: 0,
        dataPoints: []
      },
      {
        descriptor: {
          name: 'nodejs-observable-counter',
          description: '',
          type: 'OBSERVABLE_COUNTER',
          unit: '',
          valueType: 1
        },
        aggregationTemporality: 1,
        dataPointType: 0,
        dataPoints: [
          {
            attributes: {},
            startTime: [ 1656687460, 406996931 ],
            endTime: [ 1656687520, 409942931 ],
            value: 0 // <---------- look here
          }
        ]
      }
    ]
  }
]

// ---------- second report ----------

[
  {
    scope: { name: 'nodejs-metrics', version: '', schemaUrl: undefined },
    metrics: [
      {
        descriptor: {
          name: 'nodejs-counter',
          description: '',
          type: 'COUNTER',
          unit: '',
          valueType: 1
        },
        aggregationTemporality: 1,
        dataPointType: 0,
        dataPoints: []
      },
      {
        descriptor: {
          name: 'nodejs-observable-counter',
          description: '',
          type: 'OBSERVABLE_COUNTER',
          unit: '',
          valueType: 1
        },
        aggregationTemporality: 1,
        dataPointType: 0,
        dataPoints: [
          {
            attributes: {},
            startTime: [ 1656687460, 406996931 ],
            endTime: [ 1656687580, 410089014 ],
            value: 1 // <---------- look here
          }
        ]
      }
    ]
  }
]

// ---------- thirdly report ----------

[
  {
    scope: { name: 'nodejs-metrics', version: '', schemaUrl: undefined },
    metrics: [
      {
        descriptor: {
          name: 'nodejs-counter',
          description: '',
          type: 'COUNTER',
          unit: '',
          valueType: 1
        },
        aggregationTemporality: 1,
        dataPointType: 0,
        dataPoints: []
      },
      {
        descriptor: {
          name: 'nodejs-observable-counter',
          description: '',
          type: 'OBSERVABLE_COUNTER',
          unit: '',
          valueType: 1
        },
        aggregationTemporality: 1,
        dataPointType: 0,
        dataPoints: [
          {
            attributes: {},
            startTime: [ 1656687460, 406996931 ],
            endTime: [ 1656687640, 415848556 ],
            value: 2 // <---------- look here
          }
        ]
      }
    ]
  }
]

[legendecas]

Thank you for reporting. This is a problem related to DeltaMetricProcessor incorrectly saves the delta value as the cumulative value history. The problem should have been fixed in #2990.

[dyladan]

Assigning @legendecas since he wrote the PR that fixes this.

[legendecas]

The fix will be released in #3065