• BC Warning: Fixed SendGridHandler to use the V3 API as V2 is now shut down, but this requires a new API key (#1952)
• Deprecated Monolog\Test\TestCase in favor of Monolog\Test\MonologTestCase (#1953)
• Added extension point for NativeMailerHandler::mail (#1948)
• Added setHandler method to BufferHandler to modify the nested handler at runtime (#1946)
• Fixed date format in ElasticsearchFormatter to use +00:00 vs +0000 tz identifiers (#1942)
• Fixed GelfMessageFormatter handling numeric context/extra keys (#1932)

Full Changelog: https://github.com/Seldaek/monolog/compare/3.8.1...3.9.0