[ES|QL] Add `METRICS` source command support

[x] Add grammar for lexer/parser (@kbn/esql-ast).
- https://github.com/elastic/kibana/pull/183803
[x] Generate METRICS AST nodes after parsing (@kbn/esql-ast).
- https://github.com/elastic/kibana/pull/184494
[x] Code syntax highlighting in Monaco (@kbn/monaco).
- https://github.com/elastic/kibana/pull/184515
[x] Add command definition in @kbn/esql-validation-autocomplete.
- https://github.com/elastic/kibana/pull/184905
[x] Add validation support in @kbn/esql-validation-autocomplete.
- https://github.com/elastic/kibana/pull/184905
[x] Add autocomplete support in @kbn/esql-validation-autocomplete.
- https://github.com/elastic/kibana/pull/187432
~~Autocomplete should show only TSDB indices~~
- Blocked by https://github.com/elastic/elasticsearch/issues/110382
[ ] Second pass through validation and autocomplete
- [ ] Show fields inside aggregation functions.
~~Add METRICS command docs~~

elastic / kibana