Add ByteStreamObserver to enable monitoring of byte stream queue size and memory usage. #10221

Job	Run time
fixup / block-fixup	11s
labels / InternalPRRequired	1s
lint / bazel	7m 10s
test (linux, ubuntu-20.04) / bazel	9m 10s
test (linux, ubuntu-20.04, -asan) / bazel	12m 11s
test (linux, ubuntu-20.04, -debug) / bazel	9m 44s
test (macOS, macos-15) / bazel	6m 39s
test (windows, windows-2022) / bazel	24m 10s
	1h 9m 16s

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