[chore] deprecate signalfxexporter max_connections warning

[atoulme]

Description:
Pendant to #16807, we now have a better way to set up the http client settings, but we need to finish deprecating the max_connections config option. This PR removes use of MaxConnections and adds a warning if it is explicitly used.

Link to tracking Issue:
#16807

Testing:
Tests pass.

Documentation:
No changes.

[runforesight]

Foresight Summary

    
Major Impacts

build-and-test-windows duration(3 seconds) has decreased 46 minutes 13 seconds compared to main branch avg(46 minutes 16 seconds).

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⭕  changelog workflow has finished in 2 seconds (8 minutes 36 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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

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⭕  build-and-test-windows workflow has finished in 3 seconds (46 minutes 13 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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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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✅  tracegen workflow has finished in 58 seconds (2 minutes 25 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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build-dev	-     🔗	N/A	See Details
publish-latest	-     🔗	N/A	See Details
publish-stable	-     🔗	N/A	See Details

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✅  check-links workflow has finished in 1 minute 20 seconds (1 minute 42 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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changed files	-     🔗	N/A	See Details
check-links	-     🔗	N/A	See Details

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✅  prometheus-compliance-tests workflow has finished in 3 minutes 15 seconds (5 minutes 56 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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

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✅  build-and-test workflow has finished in 36 minutes 47 seconds (11 minutes 25 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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Job	Failed Steps	Tests
unittest-matrix (1.19, internal)	-     🔗	✅ 618  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, internal)	-     🔗	✅ 618  ❌ 0  ⏭ 0    🔗	See Details
correctness-metrics	-     🔗	✅ 2  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, processor)	-     🔗	✅ 1469  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, extension)	-     🔗	✅ 528  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, extension)	-     🔗	✅ 528  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, processor)	-     🔗	✅ 1469  ❌ 0  ⏭ 0    🔗	See Details
correctness-traces	-     🔗	✅ 17  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-0)	-     🔗	✅ 2564  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-0)	-     🔗	✅ 2564  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, exporter)	-     🔗	✅ 2462  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, exporter)	-     🔗	✅ 2462  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, other)	-     🔗	✅ 4434  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, other)	-     🔗	✅ 4434  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-1)	-     🔗	✅ 1887  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-1)	-     🔗	✅ 1887  ❌ 0  ⏭ 0    🔗	See Details
integration-tests	-     🔗	✅ 55  ❌ 0  ⏭ 0    🔗	See Details
setup-environment	-     🔗	N/A	See Details
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build-examples	-     🔗	N/A	See Details
lint	-     🔗	N/A	See Details
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cross-compile (darwin, amd64)	-     🔗	N/A	See Details
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cross-compile (windows, 386)	-     🔗	N/A	See Details
cross-compile (windows, amd64)	-     🔗	N/A	See Details
build-package (deb)	-     🔗	N/A	See Details
build-package (rpm)	-     🔗	N/A	See Details
windows-msi	-     🔗	N/A	See Details
publish-check	-     🔗	N/A	See Details
publish-stable	-     🔗	N/A	See Details
publish-dev	-     🔗	N/A	See Details

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✅  load-tests workflow has finished in 8 minutes 15 seconds (8 minutes 22 seconds less than main branch avg.) and finished at 6th Jan, 2023.

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

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