(core) Log number of rows in user tables in data engine

Summary:
Adds a method Table._num_rows using an empty lookup map column.

Adds a method Engine.count_rows which adds them all up.

Returns the count after applying user actions to be logged by ActiveDoc.

Test Plan: Added a unit test in Python. Tested log message manually.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3275

app/common/ActionBundle.ts

@@ -61,6 +61,7 @@ export interface SandboxActionBundle {
   calc: Array<EnvContent<DocAction>>;
   undo: Array<EnvContent<DocAction>>;   // Inverse actions for all 'stored' actions.
   retValues: any[];                     // Contains retValue for each of userActions.
+  rowCount: number;
 }
 
 // Local action that's been applied. It now has an actionNum, and includes doc actions packaged

app/server/lib/ActiveDoc.ts

@@ -1216,6 +1216,10 @@ export class ActiveDoc extends EventEmitter {
       }
       const user = docSession ? await this._granularAccess.getCachedUser(docSession) : undefined;
       sandboxActionBundle = await this._rawPyCall('apply_user_actions', normalActions, user?.toJSON());
+      log.rawInfo('Sandbox row count', {
+        ...this.getLogMeta(docSession),
+        rowCount: sandboxActionBundle.rowCount
+      });
       await this._reportDataEngineMemory();
     } else {
       // Create default SandboxActionBundle to use if the data engine is not called.
@@ -1736,7 +1740,8 @@ function createEmptySandboxActionBundle(): SandboxActionBundle {
     direct: [],
     calc: [],
     undo: [],
-    retValues: []
+    retValues: [],
+    rowCount: 0,
   };
 }