Added the stream record envelope to the public API.

temporal/api/stream/v1/message.proto

@@ -0,0 +1,48 @@
+syntax = "proto3";
+
+package temporal.api.stream.v1;
+
+option go_package = "go.temporal.io/api/stream/v1;stream";
+option java_package = "io.temporal.api.stream.v1";
+option java_multiple_files = true;
+option java_outer_classname = "MessageProto";
+option ruby_package = "Temporalio::Api::Stream::V1";
+option csharp_namespace = "Temporalio.Api.Stream.V1";
+
+import "temporal/api/common/v1/message.proto";
+
+// One entry in a stream. The record is the wire format: stores keep it
+// serialized as is and readers in every language decode the same bytes.
+message StreamRecord {
+    // The value the producer published, stored as sent. A stream provider
+    // writes and reads it as part of the record, so applying a payload codec
+    // to it is the provider's job.
+    temporal.api.common.v1.Payload body = 1;
+    // Producer-supplied provenance, stored as sent.
+    map<string, temporal.api.common.v1.Payload> metadata = 2;
+    // Producer-supplied grouping label, stored as sent.
+    string topic = 3;
+    // How to read this record. Unspecified is read as DATA.
+    StreamRecordKind kind = 4;
+    // Who wrote the record. Empty when the owning Workflow did.
+    string producer_id = 5;
+    // The producer's attempt. Readers treat a later attempt by the same
+    // producer as superseding what the earlier one wrote.
+    int64 attempt = 6;
+    // The producer's position within its attempt, zero when it does not number
+    // its records. Stored as sent; the server does not assign, validate or
+    // order by it, and the stream's own offsets are what order a read.
+    int64 sequence = 7;
+}
+
+// What a record means to a reader. Kept on the record itself so every store
+// and every language reads it the same way without a private envelope.
+enum StreamRecordKind {
+    // Read as DATA.
+    STREAM_RECORD_KIND_UNSPECIFIED = 0;
+    // A value the producer published; `body` carries it.
+    STREAM_RECORD_KIND_DATA = 1;
+    // The producer named by `producer_id` writes nothing more on `topic`.
+    // Says nothing about that producer's outcome and does not end the stream.
+    STREAM_RECORD_KIND_FINISH = 2;
+}