The proto encoding for structured logging currently pessimistically assumes each numeric tag value for encoded fields could be up to `UINT64_MAX`.

In practice, the tag values we care about are all < 16, which only requires 1 byte
of buffer space.

This CL improves the memory consumption by specifying the tag value when
calculating the buffer size needed for the structured logging proto.

PiperOrigin-RevId: 696118135
Change-Id: Iee67deef568cb4df7646d3ddd40c14b490ca0e45