Ensure all streams get synchronized in FormulaEngine

In cases where there are multiple streams lagging by the same number
of steps, only the one processed the last was getting synchronized to
the match the latest timestamp.

With this commit, `metrics_by_ts` stores a list of names for each
timestamp, so that each of them can get synchronized to the latest.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/sdk/timeseries/logical_meter/_formula_engine.py

@@ -91,19 +91,20 @@ async def _synchronize_metric_timestamps(
             name = metric.get_name()
             if result is None:
                 raise RuntimeError(f"Stream closed for component: {name}")
-            metrics_by_ts[result.timestamp] = name
+            metrics_by_ts.setdefault(result.timestamp, []).append(name)
         latest_ts = max(metrics_by_ts)
 
         # fetch the metrics with non-latest timestamps again until we have the values
         # for the same ts for all metrics.
-        for metric_ts, name in metrics_by_ts.items():
+        for metric_ts, names in metrics_by_ts.items():
             if metric_ts == latest_ts:
                 continue
-            fetcher = self._metric_fetchers[name]
             while metric_ts < latest_ts:
-                next_val = await fetcher.fetch_next()
-                assert next_val is not None
-                metric_ts = next_val.timestamp
+                for name in names:
+                    fetcher = self._metric_fetchers[name]
+                    next_val = await fetcher.fetch_next()
+                    assert next_val is not None
+                    metric_ts = next_val.timestamp
             if metric_ts > latest_ts:
                 raise RuntimeError(
                     "Unable to synchronize resampled metric timestamps, "