refactor(query-engine): split enable_topk into limiting vs formatting flags

[akanksha-akkihal]

Split enable_topk into limiting vs formatting flags

Background

Previously, enable_topk was responsible for two unrelated behaviors:

• Limiting → using the sketch heap to select top-k results (sort + truncate)
• Formatting → prepending the metric name (__name__) in PromQL-style output

This coupling worked for PromQL (which requires both behaviors), but it breaks SQL semantics.

SQL only needs top-k selection, not PromQL-style label formatting. With a single flag, it was impossible to express:

“Use heap-driven top-k selection without applying PromQL metric name formatting.”

---

Problem

The original design conflated two independent concerns:

• How results are selected (ranking / top-k computation)
• How results are represented (PromQL vs SQL output shape)

This caused SQL queries to unintentionally inherit PromQL formatting.

---

Solution: Split into two flags

We introduce two independent flags:

1. enable_topk_limiting

Controls heap-based top-k selection:

• Uses sketch heap to identify top-k elements
• Applies sorting + truncation logic

2. enable_topk_formatting

Controls PromQL-specific output formatting:

• Prepends metric name (__name__)
• Produces PromQL instant-vector style output

---

Resulting behavior

Query Type	enable_topk_limiting	enable_topk_formatting	Behavior
PromQL	true	true	Top-k + metric prefix formatting
SQL top-k	true	false	Top-k only (clean rows)
SQL/Elastic (non-top-k)	false	false	Standard execution

---

Subtle detail

Sorting remains enabled for Statistic::Topk regardless of formatting.

• Ensures correct descending order before truncation
• Prevents PromQL formatting logic from influencing SQL result structure

---

TL;DR

This change decouples:

• How we select top-k rows (ranking logic)
  from
• How results are formatted (PromQL vs SQL representation)

This separation aligns the execution model with the fundamental difference between SQL and PromQL semantics.

[milindsrivastava1997]

Can you check if there are tests that exercise PromQL's topk behavior? both in terms of the semantics, as well as the extra output label that is inserted into the result? If those tests are not there, could you please add them (use main as a source for adding them), and then test them against this branch?
Apart from that, looks good

[akanksha-akkihal]

Currently, there are no tests that exercise PromQL's topk behavior. Shall I add the test to the same PR?