feat(metrics): add configurable system prefix and separator

[Mallets]

Summary

• Add optional system: support to counter!, gauge!, and histogram! so callers can override the system prefix in metric key names.
• Add configurable separator: support for base metric declarations in the same three macros.
• Keep full backward compatibility by defaulting to SYSTEM ("quickwit") and SEPARATOR ("_") when omitted.
• Add key-name coverage tests for system/subsystem combinations and custom separators across counter/gauge/histogram, plus regression tests for single-evaluation semantics.

Metric key name composition

{system}{separator}{subsystem}{separator}{name}

Separators are inserted only between non-empty segments.
Examples:

• system: "quickwit", subsystem: "http", separator: "_" => quickwit_http_requests_total
• system: "myapp", subsystem: "http", separator: "." => myapp.http.requests_total
• system: "", subsystem: "http" => http_requests_total

Macro signatures (base declarations)

// Explicit system/subsystem with optional custom separator.
counter!(
    name: "requests_total",
    description: "...",
    system: "myapp",
    subsystem: "http",
    separator: ".",
);

// Defaults: system = SYSTEM ("quickwit"), separator = SEPARATOR ("_").
counter!(name: "requests_total", description: "...", subsystem: "http");

// Same behavior for gauge! and histogram!.

Test plan

• cargo test -p quickwit-metrics
• key-name tests cover custom/default/empty/const system cases for counter/gauge/histogram
• key-name tests cover custom separator (".") for counter/gauge/histogram
• regression tests assert inline parent label expressions are evaluated once in counter/gauge/histogram

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