Prepare v0.51.0 release (#5804)

Signed-off-by: Johan Fylling <johan.dev@fylling.se>

CHANGELOG.md

@@ -3,7 +3,67 @@
 All notable changes to this project will be documented in this file. This
 project adheres to [Semantic Versioning](http://semver.org/).
 
-## Unreleased
+## 0.51.0
+
+This release contains improvements to monitoring and an assortment of fixes and improvements.
+
+### Monitoring
+
+#### Surface unauthorized request count from OPA HTTP API authz handler via Status API
+
+Currently when OPA's HTTP server rejects requests per
+the [authz policy](https://www.openpolicyagent.org/docs/latest/security/#authentication-and-authorization), 
+this is not accounted for via the management APIs.
+This change adds that count in the metric registry that is
+part of the Status API for more visibility.
+
+([#3378](https://github.com/open-policy-agent/opa/issues/3378)) authored by @ashutosh-narkar.
+
+#### Surface more decision log errors via Status API 
+
+Previously in [5732](https://github.com/open-policy-agent/opa/pull/5732), 
+we updated the decision log plugin to
+surface errors via the Status API. However, in that change
+certain events like encoder errors and log drops due to
+buffer size limits had no metrics associated with them.
+This change adds more metrics for these events so that they
+can be surfaced via the Status API.
+
+([#5637](https://github.com/open-policy-agent/opa/issues/5637)) authored by @ashutosh-narkar.
+
+#### Include truncated HTTP response in logs 
+
+This change updates the client debug log to include
+the full HTTP response in case of non-200 status codes.
+Recording the response in the logs can help to provide
+more information to debug error scenarios.
+
+([#2961](https://github.com/open-policy-agent/opa/issues/2961)) authored by @ashutosh-narkar reported by @gshively11.
+
+### Topdown and Rego
+
+- Wasm: Add native support for `object.union_n` built-in function (authored by @Azanul)
+
+### Fixes
+
+- ast: Properly set the reported location of unused variables in strict-mode errors. ([#5662](https://github.com/open-policy-agent/opa/issues/5662)) authored by @boranx
+- fmt: report wrong arity for built-in functions. ([#5646](https://github.com/open-policy-agent/opa/issues/5646)) authored by @Trolloldem
+- topdown: http.send(): Ensuring intra-query caching consistency. ([#5736](https://github.com/open-policy-agent/opa/issues/5736)) authored by @johanfylling
+- Performance improvements to decision logging.
+  Specifically, by removing superfluous json encoding roundtrip and double work in AST conversion of to-be-logged events. (authored by @srenatus)
+
+### Docs, Website, and Ecosystem
+
+- Fix typo in documentation (authored by @eternaltyro)
+- Update TLS authentication docs (authored by @charlieegan3)
+- Clarification in docs about checksums of Windows executables (authored by @Ronnie-personal)
+- docs: Small fix to context placement in integration (authored by @craigpastro)
+- docs/website: Fix floating navbar anchor issue ([5774](https://github.com/open-policy-agent/opa/issues/5774)) authored by @charlieegan3 reported by @kristiansvalland
+
+### Miscellaneous
+
+- Update -debug images to use Chainguard images ([5544](https://github.com/open-policy-agent/opa/issues/5544)) (authored by @charlieegan3)
+- Various third-party dependencies were updated.
 
 ## 0.50.2