roachpb: add proto changes for StoreCapacity

This commit adds MMA-related proto changes to StoreCapacity, including the
writes_bytes_per_second field. It is the aggregated sum of average number of
bytes written per second across all replicas on the store. This field is
currently unused and will be populated in future commits. MMA will later use it
to track write bandwidth usage.

Epic: none
Release note: none

Author: wenyihu6

pkg/roachpb/metadata.proto

@@ -321,6 +321,13 @@ message StoreCapacity {
   // io_threshold_max tracks the maximum io overload values the store has had
   // over the last 5 minutes.
   optional cockroach.util.admission.admissionpb.IOThreshold io_threshold_max = 15 [(gogoproto.nullable) = false, (gogoproto.customname) = "IOThresholdMax" ];
+  // writes_bytes_per_second aggregates the average number of bytes written per
+  // second across replicas in the store. The stat is tracked over the time
+  // period defined in storage/replica_stats.go, which as of July 2018 is 30
+  // minutes.
+  // TODO(wenyihu6): This field is currently unused and not populated. MMA will
+  // later use it to track write bandwidth usage.
+  optional double write_bytes_per_second = 16 [(gogoproto.nullable) = false];
   // bytes_per_replica and writes_per_replica contain percentiles for the
   // number of bytes and writes-per-second to each replica in the store.
   // This information can be used for rebalancing decisions.