docs: mark matching API and related features as alpha (#16210)

Marks the matching API more clearly as experimental and updates the composite filter security posture to reflect this.

Signed-off-by: Snow Pettersen <snowp@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ b6039234e526eeccdf332a7eb041729aaa1bc286

STYLE.md

@@ -34,6 +34,11 @@ In addition, the following conventions should be followed:
   implementation. These indicate that the entity is not implemented in Envoy and the entity
   should be hidden from the Envoy documentation.
 
+* Use a `[#alpha:]` annotation in comments for messages that are considered alpha
+  and are not subject to the threat model. This is similar to the work-in-progress/alpha tagging
+  of extensions described below, but allows tagging messages that are used as part of the core API
+  as alpha without having to break it into its own file.
+
 * Always use plural field names for `repeated` fields, such as `filters`.
 
 * Due to the fact that we consider JSON/YAML to be first class inputs, we cannot easily change a

envoy/config/common/matcher/v3/matcher.proto

@@ -22,7 +22,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // As an on_no_match might result in another matching tree being evaluated, this process
 // might repeat several times until the final OnMatch (or no match) is decided.
 //
-// This API is a work in progress.
+// [#alpha:]
 message Matcher {
   // What to do if a match is successful.
   message OnMatch {

envoy/config/common/matcher/v4alpha/matcher.proto

@@ -23,7 +23,7 @@ option (udpa.annotations.file_status).package_version_status = NEXT_MAJOR_VERSIO
 // As an on_no_match might result in another matching tree being evaluated, this process
 // might repeat several times until the final OnMatch (or no match) is decided.
 //
-// This API is a work in progress.
+// [#alpha:]
 message Matcher {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.common.matcher.v3.Matcher";

envoy/extensions/common/matching/v3/extension_matcher.proto

@@ -18,6 +18,8 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // Wrapper around an existing extension that provides an associated matcher. This allows
 // decorating an existing extension with a matcher, which can be used to match against
 // relevant protocol data.
+//
+// [#alpha:]
 message ExtensionWithMatcher {
   // The associated matcher.
   config.common.matcher.v3.Matcher matcher = 1 [(validate.rules).message = {required: true}];

envoy/extensions/common/matching/v4alpha/extension_matcher.proto

@@ -19,6 +19,8 @@ option (udpa.annotations.file_status).package_version_status = NEXT_MAJOR_VERSIO
 // Wrapper around an existing extension that provides an associated matcher. This allows
 // decorating an existing extension with a matcher, which can be used to match against
 // relevant protocol data.
+//
+// [#alpha:]
 message ExtensionWithMatcher {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.extensions.common.matching.v3.ExtensionWithMatcher";

envoy/extensions/filters/http/composite/v3/composite.proto

@@ -26,6 +26,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // :ref:`ExecuteFilterAction <envoy_v3_api_msg_extensions.filters.http.composite.v3.ExecuteFilterAction>`)
 // which filter configuration to create and delegate to.
 //
+// [#alpha:]
 message Composite {
 }