Allow logging of partial raw data

[rmarx]

Currently, we use the raw field in multiple events to allow logging of the raw data.

It would be interesting to allow logging for only the first x bytes, which can help in debugging.

Some options:

1. New raw_partial field, containing the partial hex-encoded data
2. New raw_length field, indicating the length of the raw field. If it doesn't match the length parameter (or equivalent), we know raw is truncated
3. No new fields: we can derive raw's length from the size of the string. Then use similar logic as 2) to detect truncation
4. No new fields, end raw field with ... at the end of the string. Perfectly human readable, bit of a hassle when decoding the bytes to e.g., ASCII + less optimal for a potential binary format

Personally, I would prefer 3. We will strongly advise AGAINST logging raw in typical usage scenarios anyway: it's only to be used for core debugging. In these latter cases, the person debugging probably knows the stack a bit more deeply and understands that it's logging just partial raw values.

cc @huitema

[rmarx]

Related to #104 and #102

[rmarx]

Solved by allowing truncated values and specifying a separate length field (so basically option 2 above).