(core) Log statistics about table sizes

Summary:
Record numbers of rows, columns, cells, and bytes of marshalled data for most calls to table_data_from_db

Export new function get_table_stats in the sandbox, which gives the raw numbers and totals.

Get and log these stats in ActiveDoc right after loading tables, before Calculate, so they are logged even in case of errors.

Tweak logging about number of tables, especially number of on-demand tables, to not only show in debug logging.

Test Plan: Updated doc regression tests, that shows what the data looks like nicely.

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3081

sandbox/grist/main.py

@@ -56,6 +56,11 @@ def run(sandbox):
 
     sandbox.register(method.__name__, wrapper)
 
+  def load_and_record_table_data(table_name, table_data_repr):
+    result = table_data_from_db(table_name, table_data_repr)
+    eng.record_table_stats(result, table_data_repr)
+    return result
+
   @export
   def apply_user_actions(action_reprs, user=None):
     action_group = eng.apply_user_actions([useractions.from_repr(u) for u in action_reprs], user)
@@ -84,12 +89,16 @@ def run(sandbox):
 
   @export
   def load_meta_tables(meta_tables, meta_columns):
-    return eng.load_meta_tables(table_data_from_db("_grist_Tables", meta_tables),
-                                table_data_from_db("_grist_Tables_column", meta_columns))
+    return eng.load_meta_tables(load_and_record_table_data("_grist_Tables", meta_tables),
+                                load_and_record_table_data("_grist_Tables_column", meta_columns))
 
   @export
   def load_table(table_name, table_data):
-    return eng.load_table(table_data_from_db(table_name, table_data))
+    return eng.load_table(load_and_record_table_data(table_name, table_data))
+
+  @export
+  def get_table_stats():
+    return eng.get_table_stats()
 
   @export
   def create_migrations(all_tables, metadata_only=False):