feat(metrics): add requests_per_prefix_total counter with handler-level test coverage

[Copilot]

Adds a configurable per-prefix Prometheus counter on /render. Each request increments requests_per_prefix_total{prefix=...} once per configured prefix directly referenced by any target metric pattern (metric == prefix, or starts with <prefix>.).

What issue is this change attempting to solve?

The per-prefix counting logic in renderHandler was untested — existing tests only covered metricRefsPrefix and initRequestPrefixes in isolation, leaving the full target-parsing → prefix-matching → counter-increment pipeline vulnerable to silent regressions.

How does this change solve the problem? Why is this the best approach?

• New config field requestsPerPrefixList []string on the API config; empty/glob entries dropped at startup via initRequestPrefixes.
• New metric requests_per_prefix_total (CounterVec with prefix label) registered in PrometheusMetrics.
• collectMatchedPrefixes(targets, configuredPrefixes) — extracted from renderHandler's inline loop into a standalone function that parses each target string, walks exp.Metrics(), and returns the matched prefix set. This makes the full pipeline unit-testable without a live HTTP handler.
• renderHandler calls collectMatchedPrefixes before its main loop, then increments the counter for each matched prefix after all targets are processed.

Extraction into collectMatchedPrefixes is the minimal change that makes the logic independently testable while keeping renderHandler readable.

How can we be sure this works as expected?

TestCollectMatchedPrefixes covers the full pipeline (target string → parser.ParseExpr → Metrics() → metricRefsPrefix) with 10 cases:

• No configured prefixes → nil result
• Single/multiple targets matching one or more prefixes
• Function expressions (e.g. sum(a.b.c.*))
• Unparseable targets silently skipped; valid targets still counted
• Dot-boundary enforcement (a.bc.d does not match prefix a.b)
• Exact match (a.b matches prefix a.b)
• Each prefix counted at most once across multiple targets