Add getDataSize() to DocumentSnapshot

This method calculates and returns the total size of the data stored in the DocumentSnapshot in bytes.

- Iterates through the protobuf `Value` messages in the `fields` map.
- Sums the `getSerializedSize()` of each `Value`.
- Returns 0 if the document does not exist (fields is null).

Author: google-labs-jules[bot]

google-cloud-firestore/src/main/java/com/google/cloud/firestore/DocumentSnapshot.java

@@ -468,4 +468,21 @@ public String toString() {
         "%s{doc=%s, fields=%s, readTime=%s, updateTime=%s, createTime=%s}",
         getClass().getSimpleName(), docRef, fields, readTime, updateTime, createTime);
   }
+
+  /**
+   * Returns the size of the data in this snapshot in bytes.
+   *
+   * @return The size of the data in bytes.
+   */
+  public int getDataSize() {
+    if (fields == null) {
+      return 0;
+    }
+
+    int totalSize = 0;
+    for (Value value : fields.values()) {
+      totalSize += value.getSerializedSize();
+    }
+    return totalSize;
+  }
 }

google-cloud-firestore/src/test/java/com/google/cloud/firestore/QueryTest.java

@@ -1521,4 +1521,23 @@ public void inequalityFiltersImplicitlyOrderedLexicographicallyWithExplicitOrder
 
     assertEquals(orderFields, query_.createImplicitOrderBy());
   }
+
+  @Test
+  public void documentSnapshotGetDataSize() {
+    // Test with an existing document
+    DocumentSnapshot snapshot = SINGLE_FIELD_SNAPSHOT;
+    int expectedSize = 0;
+    for (Value value : snapshot.getProtoFields().values()) {
+      expectedSize += value.getSerializedSize();
+    }
+    assertEquals(expectedSize, snapshot.getDataSize());
+
+    // Test with a non-existent document
+    DocumentSnapshot missingSnapshot =
+        DocumentSnapshot.fromMissing(
+            firestoreMock,
+            firestoreMock.document("coll/doc"),
+            Timestamp.now());
+    assertEquals(0, missingSnapshot.getDataSize());
+  }
 }