Fix inconsistent field flattening

[fiinnnn]

Hey, first-time contributor here.

I noticed log fields weren't being flattened consistently and kept randomly changing on every refresh.

After looking into it, I noticed the flatten function isn't tracking depth correctly: currentDepth is incremented before every recursive call but never decremented.
As a result, after 10 steps, any further fields don't get flattened, and since map iteration order is random, the result differs per log line and per request.

This fixes the issue by decrementing currentDepth after returning from the recursive call.
Alternatively, it might be worth considering if the depth limit is really needed here.
The function is only ever used to flatten JSON objects, so an infinite loop would be extremely unlikely/impossible.

Fixes #137

[fmassot]

Looks good. I will add another test which reproduces the issue. Thanks @fiinnnn