Upgrade eslint-plugin-matrix-org to 1.2.1

This upgrade came with a number of new lints that needed to be fixed across the code base. Primarily: explicit return types on functions, and explicit visibility modifiers on class members.

src/otel/ObjectFlattener.ts

@@ -45,9 +45,9 @@ export class ObjectFlattener {
     return flatObject;
   }
 
-  static flattenSummaryStatsReportObject(
+  public static flattenSummaryStatsReportObject(
     statsReport: GroupCallStatsReport<SummaryStatsReport>
-  ) {
+  ): Attributes {
     const flatObject = {};
     ObjectFlattener.flattenObjectRecursive(
       statsReport.report,