# Support filtering/querying by JSON field in the filter bar

Follow-up to the JSON log ingestion design decision (repowise decision fdb47034ad954e3ea0b898fdc2750c32).

Extend the filter query language with a JSON field accessor (e.g. `json[field] <op> value`), comparable to the existing `metric[param] <op> value` comparison, so JSON fields can be filtered without first declaring a metric/param for them.

## Acceptance criteria
- `filter_parser.pest` gains a `json[field]` (or similar) accessor usable anywhere `metric[param]` is today, for comparisons.
- The engine evaluates `json[field]` against a line's parsed JSON (when it parses as an object), consistent with how declared params/metrics are evaluated today.
- Lines that aren't valid JSON simply don't match any `json[field]` comparison (no error).
- Existing filter syntax (metric/param comparisons, regex(), sequence, timestamp, and/or) is unaffected.