Add the span_id_tag_name field to trace schema (#99)

Author: ButterBright

README.md

@@ -351,6 +351,7 @@ BanyandbDatabase.Trace trace = BanyandbDatabase.Trace.newBuilder()
     // Mandatory identifiers
     .setTraceIdTagName("trace_id")
     .setTimestampTagName("start_time")
+    .setSpanIdTagName("span_id")
     .build();
 client.define(trace);
 ```

src/main/proto/banyandb/v1/banyandb-database.proto

@@ -244,6 +244,8 @@ message Trace {
   string timestamp_tag_name = 4 [(validate.rules).string.min_len = 1];
   // updated_at indicates when the trace resource is updated.
   google.protobuf.Timestamp updated_at = 5;
+  // span_id_tag_name is the name of the tag that stores the span ID.
+  string span_id_tag_name = 6 [(validate.rules).string.min_len = 1];
 }
 
 message StreamRegistryServiceCreateRequest {