Minor cleanups to Metrics API docs (#1695) #4795

Job	Run time
coverage	3m 16s
cargo-deny	28s
docs	1m 25s
external-types (opentelemetry)	1m 2s
external-types (opentelemetry-sdk)	1m 10s
lint	3m 48s
msrv	3m 8s
external-types (opentelemetry-otlp)	1m 18s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	38s
external-types (opentelemetry-zipkin)	1m 12s
test (stable)	6m 10s
test (beta)	6m 28s
	30m 3s

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