exporter/datadogexporter: add support for metrics as APM Stats

[gbbr]

This change adds the APM Stats consumer support into the Datadog Exporter.

Required by #16853

[runforesight]

Foresight Summary

    
Major Impacts

TestConsumeLogsUnexpectedExporterType ❌ failed 6 times in 6 runs (100% fail rate).

TestConsumeTracesUnexpectedExporterType ❌ failed 6 times in 6 runs (100% fail rate).

TestLoadInvalidConfig_NoScrapers ❌ failed 6 times in 6 runs (100% fail rate).

build-and-test-windows duration(5 seconds) has decreased 39 minutes 41 seconds compared to main branch avg(39 minutes 46 seconds).

View More Details

⭕  build-and-test-windows workflow has finished in 5 seconds (39 minutes 26 seconds less than main branch avg.) and finished at 12th Dec, 2022.

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Job	Failed Steps	Tests
windows-unittest-matrix	-     🔗	N/A	See Details
windows-unittest	-     🔗	N/A	See Details

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✅  check-links workflow has finished in 3 minutes 46 seconds (⚠️ 54 seconds more than main branch avg.) and finished at 12th Dec, 2022.

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Job	Failed Steps	Tests
changed files	-     🔗	N/A	See Details
check-links	-     🔗	N/A	See Details

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✅  tracegen workflow has finished in 4 minutes 12 seconds (⚠️ 52 seconds more than main branch avg.) and finished at 12th Dec, 2022.

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Job	Failed Steps	Tests
build-dev	-     🔗	N/A	See Details
publish-latest	-     🔗	N/A	See Details
publish-stable	-     🔗	N/A	See Details

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✅  changelog workflow has finished in 5 minutes 20 seconds and finished at 12th Dec, 2022.

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Job	Failed Steps	Tests
changelog	-     🔗	N/A	See Details

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 build-and-test workflow has finished in 23 minutes 58 seconds (38 minutes 23 seconds less than main branch avg.) and finished at 12th Dec, 2022. There are 6 test failures.

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Job	Failed Steps	Tests
unittest-matrix (1.19, internal)	N/A	✅ 592  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, internal)	N/A	✅ 592  ❌ 0  ⏭ 0    🔗	See Details
correctness-traces	N/A	✅ 17  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, extension)	N/A	✅ 528  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-0)	N/A	✅ 2532  ❌ 1  ⏭ 0    🔗	See Details
unittest-matrix (1.19, processor)	N/A	✅ 1465  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, processor)	N/A	✅ 1465  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, other)	N/A	✅ 4363  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-1)	N/A	✅ 1854  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-0)	N/A	✅ 2532  ❌ 1  ⏭ 0    🔗	See Details
integration-tests	N/A	✅ 53  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, exporter)	N/A	✅ 2237  ❌ 2  ⏭ 0    🔗	See Details
unittest-matrix (1.18, extension)	N/A	✅ 528  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, exporter)	N/A	✅ 2237  ❌ 2  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-1)	N/A	✅ 1854  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, other)	N/A	✅ 4363  ❌ 0  ⏭ 0    🔗	See Details
correctness-metrics	N/A	✅ 2  ❌ 0  ⏭ 0    🔗	See Details

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✅  prometheus-compliance-tests workflow has finished in 7 minutes 7 seconds (1 minute 59 seconds less than main branch avg.) and finished at 12th Dec, 2022.

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Job	Failed Steps	Tests
prometheus-compliance-tests	-     🔗	✅ 21  ❌ 0  ⏭ 0    🔗	See Details

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✅  load-tests workflow has finished in 11 minutes 41 seconds (4 minutes 18 seconds less than main branch avg.) and finished at 12th Dec, 2022.

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Job	Failed Steps	Tests
loadtest (TestMetric10kDPS|TestMetricsFromFile)	-     🔗	✅ 6  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestTraceNoBackend10kSPS|TestTrace1kSPSWithAttrs)	-     🔗	✅ 8  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestMetricResourceProcessor|TestTrace10kSPS)	-     🔗	✅ 12  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestTraceBallast1kSPSWithAttrs|TestTraceBallast1kSPSAddAttrs)	-     🔗	✅ 10  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestBallastMemory|TestLog10kDPS)	-     🔗	✅ 19  ❌ 0  ⏭ 0    🔗	See Details
setup-environment	-     🔗	N/A	See Details
loadtest (TestIdleMode)	-     🔗	✅ 1  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestTraceAttributesProcessor)	-     🔗	✅ 3  ❌ 0  ⏭ 0    🔗	See Details

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[mx-psi]

LGTM, just left two non-blocking comments

[mx-psi]

Should we add a changelog here, since this adds functionality visible by users? Presence/absence of this feature may be relevant on setups with multiple Collectors with different versions

[gbbr]

PTAL. The way I see things, this change is useless without the processor. If the processor is added, then that will be in the changelog, and it will signal that there is support for it, if that makes sense.

[mx-psi]

To make the situation I was referring to clearer, let's assume we release this on v0.67.0. Then the following setup will be broken:

graph LR
    subgraph Collector v0.67.0
        A[OTLP<br/> Receiver] --> B[Datadog <br/>Processor] --> C[OTLP<br/> Exporter]
    end

    subgraph Collector v0.60.0
        C --> D[OTLP<br/> Receiver] --> E[Datadog <br/>exporter]
    end

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and it will be easier for users to know why if we have a changelog that indicates what is the first version of the exporter that had support for this, the same way you added a changelog note on the Datadog Agent.