Minor cleanups to Metrics API docs #4794

Job	Run time
docs	1m 26s
cargo-deny	29s
external-types (opentelemetry)	1m 0s
coverage	3m 28s
external-types (opentelemetry-sdk)	1m 9s
external-types (opentelemetry-otlp)	1m 27s
lint	3m 32s
msrv	3m 9s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	45s
test (stable)	6m 21s
test (beta)	6m 31s
external-types (opentelemetry-zipkin)	1m 11s
	30m 28s

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