Update change log for 4.18.0-beta release (Azure#23512)

* update changelog

* update'

* update release date

Co-authored-by: annie-mac <[email protected]>

Author: xinlian12

sdk/cosmos/azure-cosmos/CHANGELOG.md

@@ -1,7 +1,18 @@
 ## Release History
 
-## 4.18.0-beta.1 (Unreleased)
+### 4.18.0-beta.1 (2021-08-11)
+#### New Features
+* Integrated cosmos diagnostics with open telemetry tracer.
+* Added `BulkExecutionOptions` and `BulkExecutionThresholds`.
 
+#### Key Bug Fixes
+* Added reactor netty timeline to query plan calls.
+* Fixed serialization warning on `clientSideRequestDiagnostics`.
+* Fixed an issue when `IdleEndpointTimeout` is 0.
+* Added retry for `PrematureCloseException`.
+* Fixed a hang issue in bulk executor.
+* Fixed an issue which preventing recovery from 410/0 after split.
+* Added TransportRequestChannelAcquisitionContext in cosmos diagnostics.
 
 ### 4.17.0 (2021-07-08)
 #### New Features