Use dedicated thread for Metrics PeriodicReader #6572

Job	Run time
lint	3m 5s
external-types (opentelemetry)	59s
msrv (windows-latest, 1.65.0)	3m 26s
external-types (opentelemetry-sdk)	1m 7s
non-default-examples (windows-latest, opentelemetry-otlp/examples/basic-otlp)	2m 5s
msrv (windows-latest, 1.71.1)	2m 9s
test (ubuntu-latest, stable)	3m 34s
external-types (opentelemetry-otlp)	1m 15s
non-default-examples (ubuntu-latest, opentelemetry-otlp/examples/basic-otlp)	38s
msrv (ubuntu-latest, 1.65.0)	2m 25s
test (ubuntu-latest, beta)	4m 22s
external-types (opentelemetry-zipkin)	1m 6s
msrv (ubuntu-latest, 1.71.1)	1m 1s
test (stable, windows-latest)	5m 24s
test (stable, macos-latest)	2m 23s
cargo-deny	26s
test (stable, actuated-arm64-4cpu-16gb)	4m 15s
coverage	2m 46s
docs	1m 3s
	43m 29s

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