return no-op span builder after shutdown

[brettmc]

this completes a todo in code comments: when tracer shared state has shutdown, getting a span builder should return a no-op implementation

[codecov]

Codecov Report

Merging #765 (93728ad) into main (f33b623) will decrease coverage by 4.34%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #765      +/-   ##
============================================
- Coverage     82.86%   78.51%   -4.35%     
- Complexity     1295     1337      +42     
============================================
  Files           150      154       +4     
  Lines          3192     3291      +99     
============================================
- Hits           2645     2584      -61     
- Misses          547      707     +160     

Flag	Coverage Δ
7.4	78.51% <100.00%> (-4.36%)	⬇️
8.0	78.59% <100.00%> (-4.34%)	⬇️
8.1	78.59% <100.00%> (-4.34%)	⬇️

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

Impacted Files	Coverage Δ
src/SDK/Trace/Tracer.php	100.00% <100.00%> (ø)
src/SDK/Trace/Span.php	12.50% <0.00%> (-81.74%)	⬇️
src/SDK/Trace/SpanLimits.php	0.00% <0.00%> (-75.00%)	⬇️
src/API/Trace/AbstractSpan.php	0.00% <0.00%> (-38.89%)	⬇️
src/SDK/Trace/Behavior/HttpSpanExporterTrait.php	93.47% <0.00%> (-1.88%)	⬇️
src/SDK/Trace/ExporterFactory.php	100.00% <0.00%> (ø)
src/SDK/Trace/SpanExporter/InMemoryExporter.php	100.00% <0.00%> (ø)
src/SDK/Common/Util/ShutdownHandler.php	63.63% <0.00%> (ø)
src/SDK/Common/Util/WeakMap.php	0.00% <0.00%> (ø)
src/SDK/Common/Util/functions.php	100.00% <0.00%> (ø)
... and 1 more

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[brettmc]

NB that I needed to adjust a bunch of tests, as tracer shared state was immediately shutting down due to their not maintaining a reference to the tracer provider. Should be fixed by #760

[tidal]

There is a huge drop in code-coverage, which is unrelated. However SpanTest has only a @coversDefaultClass annotation, which is never used, so all those tests are completely ignored by code-coverage.