feat(otlp): support INSECURE environment variables for gRPC #11188

Job	Run time
docs	1m 35s
coverage	2m 37s
msrv (ubuntu-latest)	3m 34s
build-examples	1m 32s
cargo-deny	1m 13s
msrv (windows-latest)	7m 20s
cargo-shear	3m 37s
lint	6m 53s
external-types (opentelemetry-zipkin)	1m 12s
external-types (opentelemetry-otlp)	1m 17s
test (stable, ubuntu-24.04-arm)	4m 18s
external-types (opentelemetry)	34s
test (stable, macos-latest)	3m 59s
test (ubuntu-latest, stable)	4m 25s
test (ubuntu-latest, beta)	4m 26s
external-types (opentelemetry-sdk)	42s
test (stable, windows-latest)	8m 15s
	57m 29s

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