BufferedTokenizerExt does not clear size marker on flush

[yaauie]

Logstash information:

Please include the following information:

1. Logstash version (e.g. bin/logstash --version): 6.x->9.x

Description of the problem including expected versus actual behavior:

When a BufferedTokenizer emits events through its flush method, the buffer is cleared but the internal state tracking the size of the buffer is not reset (link), resulting in subsequent use risking exceptions related to overfilling the buffer's size limit.

This is especially true for cases where events are only emitted through the flush method (e.g., sending json blobs without newline termination to the json_lines codec), in which case the metadata is never reset.

[jsvd]

fixed by #16760