victoriametrics: fix seriesByTag name tag mapping and quote-aware tag splitting

[rassvetalov]

Summary

Two fixes for seriesByTag(...) conversion to PromQL:

1. VictoriaMetrics: map name tag to __graphite__ instead of __name__.
   VictoriaMetrics stores Graphite metric paths in the __graphite__ label. The non-seriesByTag code path already uses {__graphite__=...} for plain targets. However, the seriesByTag branch used strings.ReplaceAll to rewrite 'name=' to '__name__=', which is incorrect for VM (selects the wrong label) and fragile (matches substrings like 'hostname='). Replaced with a new SeriesByTagToPromQLWithRenames helper that renames tags at the parsed-map level.

2. Quote-aware comma splitting in SplitTagValues.
   SplitTagValues used strings.Split(query, ",") which incorrectly splits tag values that contain commas (e.g. seriesByTag('desc=hello, world')). Added splitWithQuotes that respects single/double-quoted regions. Also added a guard against panics on malformed (too-short) segments.

Changes

• zipper/protocols/prometheus/helpers/helpers.go:
  • Add splitWithQuotes(s, delimiter) — quote-aware string splitter.
  • SplitTagValues now uses splitWithQuotes and skips segments shorter than 2 chars.
  • Add SeriesByTagToPromQLWithRenames(step, target, tagRenames) — applies a tag rename map after parsing, before building PromQL. The original SeriesByTagToPromQL is preserved as a zero-rename wrapper (fully backward-compatible).
• zipper/protocols/victoriametrics/fetch.go:
  • Replace strings.ReplaceAll(target.name, "'name=", "'__name__=") + SeriesByTagToPromQL with SeriesByTagToPromQLWithRenames(..., {"name": "__graphite__"}).
• zipper/protocols/prometheus/helpers/helpers_test.go:
  • Add TestSplitTagValues (simple, comma-in-value, empty, single tag).
  • Add TestSeriesByTagToPromQLWithRenames (name to graphite rename, hostname unaffected, nil renames).

Backward compatibility

• SeriesByTagToPromQL signature and behavior unchanged. All existing callers (Prometheus protocol) are unaffected.
• Only the VictoriaMetrics Fetch path changes behavior: name tag now maps to __graphite__ instead of __name__.

Test plan

• Existing helpers_test.go tests pass.
• New TestSplitTagValues covers comma-in-value edge case.
• New TestSeriesByTagToPromQLWithRenames covers rename logic and no-false-positive on similar tag names (e.g. hostname).
• Manual test with VictoriaMetrics + Graphite ingestion: seriesByTag('name=servers.web.cpu') should produce {__graphite__="servers.web.cpu"}.

[Copilot]

Pull request overview

This PR fixes two issues in the VictoriaMetrics seriesByTag(...) to PromQL conversion:

1. VictoriaMetrics name tag mapping: Changes the Graphite name tag to map to VictoriaMetrics' __graphite__ label instead of __name__. Previously used a fragile strings.ReplaceAll approach that could incorrectly match substrings like 'hostname='.

2. Quote-aware comma splitting: Implements a new splitWithQuotes function to correctly handle tag values containing commas (e.g., 'desc=hello, world'). The previous strings.Split implementation would incorrectly split on commas within quoted values.

Changes:

• Introduces SeriesByTagToPromQLWithRenames as a new public API that applies tag name transformations
• Refactors SeriesByTagToPromQL to delegate to a new internal convertSeriesByTagToPromQL function
• Adds splitWithQuotes helper for quote-aware string splitting
• Adds safety check in SplitTagValues to skip malformed segments

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
zipper/protocols/victoriametrics/fetch.go	Replaces fragile string replacement with proper tag renaming using SeriesByTagToPromQLWithRenames to map name → __graphite__; includes minor style improvement
zipper/protocols/prometheus/helpers/helpers.go	Adds quote-aware splitting, new rename API, and safety guards; includes style improvements for string comparisons
zipper/protocols/prometheus/helpers/helpers_test.go	Adds tests for new split and rename functionality

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[deniszh]

LGTM, merging this!
Thanks a lot for your contribution, @rassvetalov !