OTLP tonic metadata from env variable #3689

Job	Run time
cargo-deny	28s
coverage	8m 31s
docs	5m 37s
external-types (opentelemetry)	1m 6s
lint	20m 32s
msrv	5m 8s
external-types (opentelemetry-sdk)	1m 12s
non-default-examples (opentelemetry-otlp/examples/external-otlp-grpcio-async-std)	5m 7s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	48s
test (stable)	16m 14s
test (beta)	15m 44s
	1h 20m 27s

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