Warn users about config.async's deprecation (#1803)

* Warn users about config.async's deprecation

* Update sentry-ruby/lib/sentry/configuration.rb

Co-authored-by: Neel Shah <neel.shah@sentry.io>

* Update changelog

Co-authored-by: Neel Shah <neel.shah@sentry.io>

CHANGELOG.md

@@ -1,3 +1,9 @@
+## Unreleased
+
+### Miscellaneous
+
+- Warn users about `config.async`'s deprecation [#1803](https://github.com/getsentry/sentry-ruby/pull/1803)
+
 ## 5.3.0
 
 ### Features

sentry-ruby/lib/sentry/configuration.rb

@@ -290,6 +290,16 @@ def dsn=(value)
     def async=(value)
       check_callable!("async", value)
 
+      log_warn <<~MSG
+
+        sentry-ruby now sends events asynchronously by default with its background worker (supported since 4.1.0).
+        The `config.async` callback has become redundant while continuing to cause issues.
+        (The problems of `async` are detailed in https://github.com/getsentry/sentry-ruby/issues/1522)
+
+        Therefore, we encourage you to remove it and let the background worker take care of async job sending.
+      It's deprecation is planned in the next major release (6.0), which is scheduled around the 3rd quarter of 2022.
+      MSG
+
       @async = value
     end