perf: Re-use empty resource instance

[PROFeNoM]

Hello 👋

While searching for low-hanging fruits to improve performance, it appeared from a profile of mine that repeated calls to ResourceInfoFactory::emptyResource would start having a non-negligible impact on the performances.

The instance - being immutable - could be re-used.

Note: I haven't added a test with this PR as imo this is already been tested through the numerous calls to ResourceInfoFactory::emptyResource

Thanks! 😃

---

FYI, this is the code I used to generate the above profile, if you care, which I ran a few dozen thousand times:

function benchOpenTelemetryAPI($tracer) {
	$span = $tracer->spanBuilder('bench.basic_scenario')->setParent(false)->startSpan();
	$span->setAttribute('foo', 'bar');
	$span->setAttribute('bar', 1);

	$spans = [];
	for ($i = 0; $i < 6; $i++) {
		$childSpan = $tracer->spanBuilder('bench.basic_scenario.child')->startSpan();
		$childSpan->setAttribute('foo', 'bar');
		$childSpan->setAttribute('bar', 1);
		$spans[] = $childSpan;
	}

	for ($i = 5; $i >= 0; $i--) {
		$spans[$i]->end();
	}

	$span->end();
}

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.61%. Comparing base (b20c45d) to head (70d8bc6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1251      +/-   ##
============================================
+ Coverage     83.09%   84.61%   +1.51%     
+ Complexity     2275     2137     -138     
============================================
  Files           285      284       -1     
  Lines          6460     6056     -404     
============================================
- Hits           5368     5124     -244     
+ Misses         1092      932     -160     

Flag	Coverage Δ
7.4	?
8.0	84.56% <66.66%> (+1.56%)	⬆️
8.1	84.59% <66.66%> (+1.43%)	⬆️
8.2	84.59% <66.66%> (+1.43%)	⬆️
8.3	84.59% <66.66%> (+1.43%)	⬆️
8.4	84.59% <66.66%> (+1.43%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/SDK/Resource/ResourceInfoFactory.php	97.56% <66.66%> (-2.44%)	⬇️

... and 157 files with indirect coverage changes

---

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[brettmc]

Thanks, @PROFeNoM :)