Various fixes & improvements

• fix(loguru): Move integration setup from __init__ to setup_once (#4399) by @sentrivana
• feat: Allow configuring keep_alive via environment variable (#4366) by @szokeasaurusrex
• fix(celery): Do not send extra check-in (#4395) by @sentrivana
• fix(typing): Add before_send_log to Experiments (#4383) by @sentrivana
• ci: Fix pyspark test suite (#4382) by @sentrivana
• fix(logs): Make sentry.message.parameters singular as per spec (#4387) by @AbhiPrasad
• apidocs: Remove snowballstemmer pin (#4379) by @sentrivana