Remove APPARENT_ from current metric names (#208)

Current metrics have been renamed:
- `METRIC_AC_APPARENT_CURRENT` -> `METRIC_AC_CURRENT`
- `METRIC_AC_APPARENT_CURRENT_PHASE_[1|2|3]` ->
`METRIC_AC_CURRENT_PHASE_[1|2|3]`

Closes #178

Author: tiyash-basu-frequenz

RELEASE_NOTES.md

@@ -22,6 +22,10 @@
 - The variants in the enum `Metric` have been renumberd due to the addition of
   line-to-line voltages.
 
+- Current metrics have been renamed:
+  `METRIC_AC_APPARENT_CURRENT` -> `METRIC_AC_CURRENT`
+  `METRIC_AC_APPARENT_CURRENT_PHASE_[1|2|3]` -> `METRIC_AC_CURRENT_PHASE_[1|2|3]`
+
 ## New Features
 
 - Added a `Frequenz.api.common.v1.types.Decimal` type, compatible with

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

@@ -81,10 +81,10 @@ enum Metric {
   METRIC_AC_VOLTAGE_PHASE_1_PHASE_2 = 15;
   METRIC_AC_VOLTAGE_PHASE_2_PHASE_3 = 16;
   METRIC_AC_VOLTAGE_PHASE_3_PHASE_1 = 17;
-  METRIC_AC_APPARENT_CURRENT = 18;
-  METRIC_AC_APPARENT_CURRENT_PHASE_1 = 19;
-  METRIC_AC_APPARENT_CURRENT_PHASE_2 = 20;
-  METRIC_AC_APPARENT_CURRENT_PHASE_3 = 21;
+  METRIC_AC_CURRENT = 18;
+  METRIC_AC_CURRENT_PHASE_1 = 19;
+  METRIC_AC_CURRENT_PHASE_2 = 20;
+  METRIC_AC_CURRENT_PHASE_3 = 21;
 
   // AC power metrics
   METRIC_AC_APPARENT_POWER = 22;