implement SpanProcessor::onEnding (#1483) #77

Job	Run time
php (8.2, false)	1m 42s
php (8.3, false)	1m 45s
php (8.4, false, --ignore-platform-reqs)	2m 9s
php (8.5, true, --ignore-platform-reqs)	1m 54s
packages / validate (8.2, ubuntu-latest, ast, json, grpc, false)	1m 3s
packages / validate (8.3, ubuntu-latest, ast, json, grpc, false)	1m 15s
packages / validate (8.4, ubuntu-latest, ast, json, grpc, false)	56s
	10m 44s

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