feat: fetch all flags rpc for sync server (#82)

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## This PR
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- adds the fetch all flags rpc for sync server, required for flagd to
resend state on merge events

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Signed-off-by: James Milligan <james@omnant.co.uk>

Author: james-milligan

protobuf/buf.md

@@ -21,12 +21,19 @@ a supported plugin for generating connect stubs then it is recommended to use th
 
 ## Flag sync
 
-The module `sync.v1` is a grpc server streaming service definition to provide flagd with feature flag configurations.
-This service exposes a single method `SyncFlags`. Flagd acts as the client and initiates the streaming with `SyncFlagsRequest`.
+The module `sync.v1` is a service definition to provide flagd with feature flag configurations.
+The server exposes 2 `rpcs`.
 
+### SyncFlags
+Flagd acts as the client and initiates the streaming with `SyncFlagsRequest`.
 The server implementation will then stream feature flag configurations through `SyncFlagsResponse`. The response contains
 `SyncState` which can be utilized to provide flagd with flexible configuration updates.
 
+### FetchAllFlags
+Flagd acts as the client and sends the empty message `FetchAllFlagsRequest`.
+The server implementation responds with the full feature flag configuration through the `FetchAllFlagsResponse`.
+This `rpc` is used to re-sync flagd's internal state during configuration merge events.
+
 ## Code generation
 
 Easiest way to generate grpc code for your language of choice is using provided [buf](https://buf.build/) templates.

protobuf/sync/v1/sync_service.proto

@@ -46,7 +46,17 @@ message SyncFlagsResponse {
   SyncState state = 2;
 }
 
+// FetchAllFlagsRequest is the request to fetch all flags. Flagd sends this request as the client in order to resync its internal state
+message FetchAllFlagsRequest {}
+
+//  FetchAllFlagsResponse is the server response containing feature flag configurations
+message FetchAllFlagsResponse {
+  // flagd feature flag configuration. Must be validated to schema - https://raw.githubusercontent.com/open-feature/schemas/main/json/flagd-definitions.json
+  string flag_configuration = 1;
+}
+
 // FlagService implements a server streaming to provide realtime flag configurations
 service FlagSyncService {
   rpc SyncFlags(SyncFlagsRequest) returns (stream SyncFlagsResponse) {}
+  rpc FetchAllFlags(FetchAllFlagsRequest) returns (FetchAllFlagsResponse) {}
 }