Add approx_topk and multi-variant expressions to LogQL parser

[btaani]

Add support for LogQL features included in Loki between v3.1.0 and v3.6.5

$ git log --oneline v3.1.0..v3.6.5 -- pkg/logql/syntax/expr.y pkg/logql/syntax/syntax.y
417d0a5252 chore: add multi-variant query support to LogQL (#16196)
1187a73667 chore: LogQL syntax and AST simplification and cosistency (#16028)
7b53f20f70 feat: Introduce shardable probabilistic topk for instant queries. (#14243)

• feat: Introduce shardable probabilistic topk for instant queries. (backport k227) grafana/loki#14765

• chore: add multi-variant query support to LogQL grafana/loki#16196

Ref: LOG-8712

[btaani]

/hold
fixing & adding more tests

[JoaoBraveCoding]

approx_topk seems to still be in experimental feature according to the docs. It works when sent directly to the querier, but fails with parse error at line 1, col 1: syntax error: unexpected IDENTIFIER when sent through query-frontend.

I verified this by calling both components directly over mTLS with the same query: the querier returned 200 and valid results, while the query-frontend returned 400. This points to a query-frontend bug... likely related to approx_topk being rewritten into internal non-LogQL expressions during frontend processing and then reparsed on a code path that expects normal LogQL.

/cc @xperimental with this shall we add support for approx_topk? I'm on the fence because I'm not sure if grafana will eventually support it or drop it... at the same time if we don't add support we would need to remember that

multi-variant worked perfectly 👌