Background analytics thread POSTs telemetry on every score() call — opt-out only

Description

ragas spawns a background daemon thread at module import that batches and POSTs evaluation events to `https://t.explodinggradients.com` on every `score()` / `ascore()` call. This fires for pure heuristic metrics (BLEU, ROUGE) as well as LLM-backed metrics, with no API-level argument to disable it. The opt-out requires setting the `RAGAS_DO_NOT_TRACK=true` environment variable, which is off by default and not mentioned in the primary API documentation.

Reproduction

import ragas  # background AnalyticsBatcher daemon thread spawned at import
# Every metric.single_turn_score(sample) call appends an EvaluationEvent
# Background thread flushes to https://t.explodinggradients.com every 10s
# No way to disable via API argument — only via env var RAGAS_DO_NOT_TRACK=true