Fix UDP Dial for go 1.5

… EVP flagevaluation contract

- #1 (blocker): route globalCap overflow to ultra-degraded instead of dropping; count total preserved at every tier
- #3: add TestSaturationCountPreservation (total counts == add() calls past both globalCap and perFlagCap)
- #2: correct collision-safe wording to low-collision/not collision-proof; document count-preserving collision behavior
- #4: rotate flag keys + distinct context field names in benchmark so real cardinality is exercised

…ook-fire time (review #3)

Capture the evaluation timestamp once at DatadogProvider.evaluate entry, thread it into
evaluateFlag (reusing the time.Now() the evaluator already computes for allocation windows),
and stamp it into FlagMetadata on every return path. The EVP flagevaluation hook reads it for
first/last_evaluation bounds instead of calling time.Now() at hook-fire time (slightly later
and slightly incorrect). Falls back to hook-fire time when absent (non-Datadog provider).
flageval_metrics.go untouched.