refactor(exporter): remove dead MetricFixedString interface method + FixedStringCol class

[JoshDreamland]

Summary

Remove the MetricFixedString interface method and the FixedStringCol class that implements it. Both became unreferenced when PR #99 changed err_sqlstate from FixedString(5) to LowCardinality(String). Pure deletion; no behavior change. -35 LOC, three files.

Why this is its own PR

MetricFixedString was added in commit 9f89f4 (Feb 25 2026, "Extract an interface for statistics reporting") as a one-liner over clickhouse::ColumnFixedString from the clickhouse-cpp library. It served exactly one column (err_sqlstate, width 5).

PR #104 (merged 2026-06-04) replaced clickhouse-cpp with clickhouse-c, which doesn't have a built-in fixed-string column class. To preserve the interface contract, the port hand-rolled a 27-line FixedStringCol class on top of clickhouse-c's lower-level AppendFixed primitive. At landing time this was correct — err_sqlstate was still FixedString(5) in the schema and the call site was still MetricFixedString(5, "err_sqlstate").

PR #99 (merged 2026-06-10, six days later) changed err_sqlstate to LowCardinality(String) because FixedString does not round-trip cleanly through the Arrow IPC pipeline that prod uses, and ~270 SQLSTATE codes are dictionary-friendly. The call site moved to TagString("err_sqlstate"). The interface method and the hand-rolled class became unreachable code in main. Nobody noticed at the time.

This PR removes both. The reasoning that pushed err_sqlstate to LowCardinality(String) in PR #99 also explains why we don't want to re-add FixedString support to clickhouse-c "for parity" — the only consumer in this codebase was already moved off it deliberately, and any future fixed-width-binary column would hit the same Arrow round-trip issue.

Why FixedString isn't the right type for SQLSTATE (in case the question comes up)

Property	FixedString(5)	LowCardinality(String) (~270 codes)
Bytes/row	5 raw bytes	2 (UInt16 dict index), shared per-part dict ~1.6 KB
Filter speed	byte compare per row	integer compare per row, dictionary-aware pushdown
Arrow IPC equivalent	FixedSizeBinary — not transparently castable to FixedString(N) on the CH side	DictionaryUtf8 — round-trips cleanly
Empty (success) value	5 NUL bytes	empty string, single dict entry

The Arrow round-trip is the deciding factor; storage and query speed both happen to favor LC too.

Test plan

• All builds pass on PG 16/17/18 × amd64/arm64 (CI)
• Code Style + TAP suite pass — no behavior change so nothing should regress
• PGXN check passes

This is a no-op refactor; the deleted code was unreachable.

🤖 Generated with Claude Code

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Note

Low Risk
Pure deletion of unreachable interface and implementation code with no remaining call sites.

Overview
Removes MetricFixedString from StatsExporter and drops the ClickHouse-only FixedStringCol implementation that backed it. ClickHouseExporter and OTelExporter no longer declare that factory method.

This is cleanup after err_sqlstate moved to TagString / LowCardinality(String); nothing in the tree still calls the fixed-width metric path. Export behavior is unchanged.

^{Reviewed by Cursor Bugbot for commit 50bca23. Bugbot is set up for automated code reviews on this repo. Configure here.}

[Copilot]

Pull request overview

Removes dead fixed-width string metric support from the exporter abstraction after err_sqlstate moved to LowCardinality(String), leaving no remaining call sites for the fixed-string API.

Changes:

• Removed MetricFixedString(int len, string_view name) from the StatsExporter interface.
• Removed now-unused overrides from OTelExporter and ClickHouseExporter.
• Deleted the ClickHouse-only FixedStringCol implementation class.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/export/exporter_interface.h	Drops the MetricFixedString pure virtual from StatsExporter.
src/export/otel_exporter.cc	Removes the MetricFixedString override (previously a string_view column).
src/export/clickhouse_exporter.cc	Removes MetricFixedString override and deletes the unused FixedStringCol column implementation.

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