Add the span_id_tag_name field to trace schema (#99)

diff --git a/README.md b/README.md
index a884345..1922d14 100644
--- a/README.md
+++ b/README.md
@@ -351,6 +351,7 @@
     // Mandatory identifiers
     .setTraceIdTagName("trace_id")
     .setTimestampTagName("start_time")
+    .setSpanIdTagName("span_id")
     .build();
 client.define(trace);
 ```
diff --git a/src/main/proto/banyandb/v1/banyandb-database.proto b/src/main/proto/banyandb/v1/banyandb-database.proto
index 5c65eff..c37031e 100644
--- a/src/main/proto/banyandb/v1/banyandb-database.proto
+++ b/src/main/proto/banyandb/v1/banyandb-database.proto
@@ -244,6 +244,8 @@
   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 {