fix(osmsg/handlers.py): --length silently produced no output unless the same key was also passed to --tags

[gauravbarall]

The bug

In the default mode (no --all, no --keys), ChangefileHandler._accumulate decided which tag keys to record stats for by checking cfg["additional_tags"] (i.e. --tags) only.

--length is documented as its own independent flag, but the code never consulted cfg["length"] when building that list. As a result, any key passed to --length that wasn't also repeated in --tags never got a TagValueStat written for it — no error, no warning, just an empty/zero result.

The fix

Track the union of --tags and --length keys instead of just --tags.

The tracked keys are computed once in __init__ as self._tracked_keys, rather than re-reading cfg["additional_tags"] for every element.

Before

uv run osmsg --length highway --last day

highway_len_m comes back as 0 for every row.

ChangefileHandler never accumulated anything for highway because --length doesn't feed into additional_tags.

After

The same command now correctly reports the summed length of created highway=* ways.

--length works independently and also correctly picks up keys that are in --length but not in --tags when both are passed together.

For example:

uv run osmsg --tags building --length highway --last day

Previously, highway was silently dropped because only keys from --tags were tracked. It is now correctly included.