Adds IndexKey::MetricDeclaration handling: DeclareMetric/RemoveMetricDeclaration
engine messages and a MetricDeclarations query, mirroring the pattern from ticket
001's param declarations. Params are derived from inline named capture groups on
the metric's own regex for v1 (referencing existing ParamDeclarations is left for
a future ticket). Validates the regex and every RegexToMetricPrimitiveMatcher's
capture-group reference at declaration time, reporting all bad matchers at once.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
Implements the ParamDeclaration slice of the primary index: DataValue::ParamDeclaration
holds a regex + named capture group + MetricPrimitiveKind, stored under
IndexKey::ParamDeclaration{name} between LogParamSplit and ParamMetricSplit.
EngineMessage gains DeclareParam (validates the regex compiles and the capture
group exists before storing, returning a clear error otherwise) and
RemoveParamDeclaration; EngineQuery gains ParamDeclarations to list them back out
as ParamDeclarationDto. No API wiring yet (separate ticket).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
