Add missing entries to CHANGELOG.md

I've had a look through all the commits that landed on `main` since
4.0.0.

I haven't included absolutely everything here - just commits which felt
user-facing enough to include in a changelog.

Signed-off-by: Chris Sinjakli <[email protected]>

Author: Sinjo

CHANGELOG.md

@@ -10,6 +10,15 @@ _**Codename:** They finally made a point release_
 
 ## Small improvements
 
+- [#264](https://github.com/prometheus/client_ruby/pull/264) Add JRuby 9.3 to build matrix:
+    JRuby 9.3 was released, and added as an officially supported version
+- [#273](https://github.com/prometheus/client_ruby/pull/273) Add Ruby 3.2 to build matrix:
+    Ruby 3.2 was released, and added as an officially supported version
+- [#280](https://github.com/prometheus/client_ruby/pull/280) Optimize incrementing values
+    in DirectFileStore adapter:
+    There were some expensive method calls being made multiple times when they didn't need
+    to be for simple increments. This PR introduces a specialised implementation for that
+    case.
 - [#277](https://github.com/prometheus/client_ruby/pull/277) Allow use of `instance` and
     `job` labels:
     It's now possible to set the `instance` and `job` labels on metrics, where previously
@@ -25,6 +34,19 @@ _**Codename:** They finally made a point release_
     the scraped metric data to take precedence over the labels generated by the
     server.
 
+## Bug fixes
+
+- [#268](https://github.com/prometheus/client_ruby/pull/268) Use lowercase response headers
+    in Rack example:
+    Rack 3.0.0 started requiring this for compatibility with HTTP/2
+- [#271](https://github.com/prometheus/client_ruby/pull/271) Use lowercase for HTTP headers
+    in middleware:
+    Fixes the same issue from above in our middleware
+- [#270](https://github.com/prometheus/client_ruby/pull/270) Small compatibility fixes in
+    Rack example:
+    Apple have taken port 5000 for AirPlay, so we had to move away from it. Go has changed
+    how you install binaries, so we updated those instructions too.
+
 # 4.0.0 / 2022-03-27
 
 _**Codename:** The "barely a release" release_