otlpmetrichttp: Use go.opentelemetry.io/proto/slim/otlp #11782

Job	Run time
lint	3m 27s
test-bench	1m 26s
test-coverage	1m 18s
test-race	2m 44s
compatibility-test (~1.22.2, ubuntu-latest, 386)	1m 35s
compatibility-test (~1.22.2, ubuntu-latest, amd64)	1m 15s
compatibility-test (~1.22.2, macos-latest, amd64)	47s
compatibility-test (~1.22.2, windows-latest, 386)	2m 49s
compatibility-test (~1.22.2, windows-latest, amd64)	2m 32s
compatibility-test (~1.22.2, actuated-arm64-2cpu-8gb, arm64)	24s
compatibility-test (~1.21.9, ubuntu-latest, 386)	1m 17s
compatibility-test (~1.21.9, ubuntu-latest, amd64)	1m 19s
compatibility-test (~1.21.9, macos-latest, amd64)	35s
compatibility-test (~1.21.9, windows-latest, 386)	3m 52s
compatibility-test (~1.21.9, windows-latest, amd64)	2m 30s
compatibility-test (~1.21.9, actuated-arm64-2cpu-8gb, arm64)	23s
test-compatibility	1s
	28m 14s

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