final T readLengthPrefixAndValue(ReadBuffer b) {
      int length = b.readVarint32();
      if (length == 0) return null;
      return readValue(b, length);
    }
Calling this after consuming the field key to ensures there's enough space for the data. Null
is returned when the length prefix is zero.
zipkin/src/main/java/zipkin2/internal/Proto3Fields.java