Add OnStoreRecordingsInReport method to UKMEntryFilter.

This method is needed so that we can leverage the filter
for rate-limiting and deduplicating metrics on a per-report
basis.

Bug: 1112787
Change-Id: I230f06eff4c00479387ad1dcc88bf51a3a2034e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363688Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/master@{#800197}

components/ukm/ukm_entry_filter.h

@@ -44,6 +44,10 @@ class UkmEntryFilter {
   virtual bool FilterEntry(
       mojom::UkmEntry* entry,
       base::flat_set<uint64_t>* removed_metric_hashes) const = 0;
+
+  // UkmService invokes this method every time a report is generated.
+  //
+  virtual void OnStoreRecordingsInReport() const {}
 };
 
 }  // namespace ukm

components/ukm/ukm_recorder_impl.cc

@@ -530,6 +530,11 @@ void UkmRecorderImpl::StoreRecordingsInReport(Report* report) {
     }
   }
   source_counts_proto->set_entryless_sources(num_sources_entryless);
+
+  // Notify observers that a report was generated.
+  if (entry_filter_) {
+    entry_filter_->OnStoreRecordingsInReport();
+  }
 }
 
 bool UkmRecorderImpl::ShouldRestrictToWhitelistedSourceIds() const {