private int decodedBufferSize(int len) {
        return (len + pos) / 4 * 3;
    }
A stateful Base64decoder. Unlike {@link io.netty.handler.codec.base64.Base64Decoder}, this decoder does
not end when it meets padding('='), but continues to decode until the end of the {@link ByteBuf} given by
{@link #decode(ByteBuf)}. If the {@link ByteBuf} does not have necessary 4 bytes to decode as 3 bytes,
it stores the remained bytes and prepend them to the next {@link #decode(ByteBuf)} and decode together.
grpc-protocol/src/main/java/com/linecorp/armeria/internal/common/grpc/protocol/Base64Decoder.java