Trim long values in AppMaps

[dividedmind]

The AppMap specification says this about value fields:

string describing the object. This is not a strict JSON serialization, but rather a display string which is intended for the user. These strings should be trimmed in length to 100 characters.

appmap-node currently does no such trimming. Note this can be particularly inconvenient when eg. this is an object of a rich class with many methods. Perhaps it's worthwhile to filter the dump to only own properties. We want to make these 100 chars count.

[kgilpin]

Yes the agent is free to implement this limit in whatever makes the most sense. 100 characters is guidance - it’s important to have some limit to avoid outputting pathologically long strings and blowing up the map files.