Add field source to MetricSample (#181)

This commit adds an optional string fieldthat can be used to identify
the source of the metric. This is useful in cases where the same metric
can be obtained from multiple sensors or sources in the same component.

E.g., a hybrid inverter can have multiple DC strings, each with its own
current and voltage sensors. In such cases, the source field can be used
to differentiate between the different sources.

Author: tiyash-basu-frequenz

RELEASE_NOTES.md

@@ -10,7 +10,9 @@
 
 ## New Features
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- Added a field named `source` to `MetricSample` to allow the user to identify
+  the source of the metric, in case different sensors in component report
+  metrics with the same `Metric` variant.
 
 ## Bug Fixes
 

proto/frequenz/api/common/v1/metrics/metric_sample.proto

@@ -201,4 +201,20 @@ message MetricSample {
   // ---- values here are disallowed and will be rejected
   // ==== values here are allowed and will be accepted
   repeated Bounds bounds = 4;
+
+  // An optional string that can be used to identify the source of the metric.
+  //
+  // This is expected to be populated when the same `Metric` variant can be
+  // obtained from multiple sensors in the component. Knowing the source of the
+  // metric can help in certain control and monitoring applications.
+  //
+  // E.g., a hybrid inverter can have a DC string for a battery and another DC
+  // string for a PV array. The source names could resemble, say,
+  // `dc_battery_0` and ``dc_pv_0`. A metric like DC voltage can be obtained
+  // from both sources. For an application to determine the SoC of the battery
+  // using the battery voltage, the source of the voltage metric is important.
+  //
+  // In cases where the component has just one source for a metric, then this
+  // field is not expected to be present, because the source is implicit.
+  optional string source = 5;
 }