feat: add sync stream type and context (#183)

This PR updates the sync schema. It:

- deprecates the
[GetMetadata](https://buf.build/open-feature/flagd/docs/main:flagd.sync.v1#flagd.sync.v1.FlagSyncService.GetMetadata)
rpc and its messages
- includes a new, **_optional_** `sync_context` field which conveys the
same data as the now-deprecated GetMetadata rpc
- `optional` makes it easy for providers to absorb this change in a
non-breaking way, since it generates code which allows us to check if
the the field is explicitly set or not, and if not, fall back to the old
GetMetadata
- ~includes a new, **_required_** `type` field, exactly equivalent to the~
`type` field in the
[EventStream](https://buf.build/open-feature/flagd/docs/main:flagd.evaluation.v1#flagd.evaluation.v1.EventStreamResponse)
- this field must contain `provider_ready` on the first stream payload,
and `configuration_change` in subsequent payloads
- this field is required, **_meaning this is a breaking change for
servers_** (they will need to be updated to check this field)
- this allows providers to commonize more logic between sync/event
stream implementations, and remove some state maintenance associated
with sync stream implementations

Relates to: open-feature/flagd#1584
Relates to: open-feature/flagd#1585

In general, I'm wondering what people think of the name `sync_context`
for the new field carrying the static context from flagd. My argument
for `sync_context` is simple, in that it carries the auxiliary context
from server and it's delivered via the sync stream. `static_context`
might also work, but it might not be "static" in some implementations
and could change in a new stream payload (though it won't in flagd).
`GetMetadata` was poorly named, and I'm hoping this is clearer.

@cupofcat I believe you have your own server implementation, so I'm
interested in your thoughts on both these changes.

---------

Signed-off-by: Todd Baert <[email protected]>

Author: toddbaert

protobuf/flagd/sync/v1/sync.proto

@@ -37,6 +37,8 @@ message SyncFlagsRequest {
 message SyncFlagsResponse {
   // flagd feature flag configuration. Must be validated to schema - https://raw.githubusercontent.com/open-feature/flagd-schemas/main/json/flags.json
   string flag_configuration = 1;
+  // Static context to be included in in-process evaluations (optional).
+  optional google.protobuf.Struct sync_context = 2;
 }
 
 // FetchAllFlagsRequest is the request to fetch all flags. Clients send this request as the client in order to resync their internal state
@@ -62,17 +64,23 @@ message FetchAllFlagsResponse {
 }
 
 // GetMetadataRequest is the request for retrieving metadata from the sync service
-message GetMetadataRequest {}
+message GetMetadataRequest {
+  option deprecated = true;
+}
 
 // GetMetadataResponse contains metadata from the sync service
+// DEPRECATED; use flagd.sync.v1.SyncFlagsResponse.sync_context
 message GetMetadataResponse {
   reserved 1; // old key/value metadata impl
   google.protobuf.Struct metadata = 2;
+  option deprecated = true;
 }
 
 // FlagService implements a server streaming to provide realtime flag configurations
 service FlagSyncService {
   rpc SyncFlags(SyncFlagsRequest) returns (stream SyncFlagsResponse) {}
   rpc FetchAllFlags(FetchAllFlagsRequest) returns (FetchAllFlagsResponse) {}
-  rpc GetMetadata(GetMetadataRequest) returns (GetMetadataResponse) {}
+  rpc GetMetadata(GetMetadataRequest) returns (GetMetadataResponse) {
+    option deprecated = true;
+  }
 }