getMessageStringBuilder() called setLength(0) which resets the logical length but never shrinks the internal char[] buffer. In thread-pool environments, a single large trace permanently bloats the buffer for that thread's lifetime, causing cumulative heap pressure.

Add a capacity threshold check (8 KB) that discards oversized buffers and replaces them with fresh instances, while still reusing normally-sized buffers for zero-allocation steady state.

Fixes elastic#381