Update Beam model protos with element metadata (#34971)

As part of https://s.apache.org/beam-element-extended-metadata we want to add ElementMetadata that will hold in the future additional metadata fields e.g. drainMode, element kind or tracing metadata.

Author: stankiewicz

model/fn-execution/src/main/proto/org/apache/beam/model/fn_execution/v1/beam_fn_api.proto

@@ -740,6 +740,12 @@ message Elements {
     bool is_last = 4;
   }
 
+  // Element metadata passed as part of WindowedValue to make WindowedValue
+  // extensible and backward compatible
+  message ElementMetadata {
+    // empty message - add drain, kind, tracing metadata in the future
+  }
+
   // Represent the encoded user timer for a given instruction, transform and
   // timer id.
   message Timers {

model/pipeline/src/main/proto/org/apache/beam/model/pipeline/v1/beam_runner_api.proto

@@ -1677,6 +1677,10 @@ message StandardProtocols {
     // Indicates whether the SDK supports ordered list state.
     ORDERED_LIST_STATE = 10
     [(beam_urn) = "beam:protocol:ordered_list_state:v1"];
+
+    // Indicates support for reading, writing and propagating Element's metadata
+    ELEMENT_METADATA = 11
+    [(beam_urn) = "beam:protocol:element_metadata:v1"];
   }
 }