fix(c/driver/postgresql): add postgres type to cols created for numeric

- introduce constant with key name
- introduce separate private method to add field-level method

c/driver/postgresql/postgres_type.h

@@ -216,9 +216,12 @@ class PostgresType {
         break;
 
       // ---- Numeric/Decimal-------------------
-      case PostgresTypeId::kNumeric:
+      case PostgresTypeId::kNumeric: {
         NANOARROW_RETURN_NOT_OK(ArrowSchemaSetType(schema, NANOARROW_TYPE_STRING));
+        NANOARROW_RETURN_NOT_OK(AddPostgresTypeMetadata(schema));
+
         break;
+      }
 
       // ---- Binary/string --------------------
       case PostgresTypeId::kChar:
@@ -284,13 +287,7 @@ class PostgresType {
         // can still return the bytes postgres gives us and attach the type name as
         // metadata
         NANOARROW_RETURN_NOT_OK(ArrowSchemaSetType(schema, NANOARROW_TYPE_BINARY));
-        nanoarrow::UniqueBuffer buffer;
-        ArrowMetadataBuilderInit(buffer.get(), nullptr);
-        NANOARROW_RETURN_NOT_OK(ArrowMetadataBuilderAppend(
-            buffer.get(), ArrowCharView("ADBC:postgresql:typname"),
-            ArrowCharView(typname_.c_str())));
-        NANOARROW_RETURN_NOT_OK(
-            ArrowSchemaSetMetadata(schema, reinterpret_cast<char*>(buffer->data)));
+        NANOARROW_RETURN_NOT_OK(AddPostgresTypeMetadata(schema));
         break;
       }
     }
@@ -309,6 +306,19 @@ class PostgresType {
   std::string typname_;
   std::string field_name_;
   std::vector<PostgresType> children_;
+
+  static constexpr const char* kPostgresTypeKey = "ADBC:postgresql:typname";
+
+  ArrowErrorCode AddPostgresTypeMetadata(ArrowSchema* schema) const {
+    nanoarrow::UniqueBuffer buffer;
+    ArrowMetadataBuilderInit(buffer.get(), nullptr);
+    NANOARROW_RETURN_NOT_OK(ArrowMetadataBuilderAppend(
+        buffer.get(), ArrowCharView(kPostgresTypeKey), ArrowCharView(typname_.c_str())));
+    NANOARROW_RETURN_NOT_OK(
+        ArrowSchemaSetMetadata(schema, reinterpret_cast<char*>(buffer->data)));
+
+    return NANOARROW_OK;
+  }
 };
 
 // Because type information is stored in a database's pg_type table, it can't