Metadata HTTP API

Summary:
An interface that exposes Metadata via a REST API.
Is mainly used for testing and not enabled by default

Only enabled with `metadata-api` build flag

Usage:
The API is served by the admin port under `/metadata/<key>`
GET, and HEAD operations always populates the `X-Version` header
if a value is found.

PUT and DELETE operations both require a `X-Precondition` header.
If the precondition is not met, the operation fails with (412 Precondition Failed)

PUT operation requires a header `X-Version` which sets the version
of the new value

crates/admin/Cargo.toml

@@ -14,6 +14,7 @@ options_schema = ["restate-service-client/options_schema"]
 memory-loglet = ["restate-bifrost/memory-loglet"]
 replicated-loglet = ["restate-bifrost/replicated-loglet"]
 serve-web-ui = ["restate-web-ui", "mime_guess"]
+metadata-api = []
 
 [dependencies]
 workspace-hack = { version = "0.1", path = "../../workspace-hack" }

crates/admin/src/lib.rs

@@ -10,6 +10,8 @@
 
 pub mod cluster_controller;
 mod error;
+#[cfg(feature = "metadata-api")]
+mod metadata_api;
 mod rest_api;
 mod schema_registry;
 pub mod service;