Allow negative bucket bounds in explicit_bounds (#163)

Fixes #156

@nilebox @bogdandrutu @jmacd

Co-authored-by: Bogdan Drutu <bogdandrutu@gmail.com>

Author: x13n

gen/go/metrics/v1/metrics.pb.go

@@ -373,14 +373,14 @@ message HistogramDataPoint {
   // This defines size(bounds) + 1 (= N) buckets. The boundaries for bucket
   // at index i are:
   //
-  // [0, bounds[i]) for i == 0
+  // (-infinity, bounds[i]) for i == 0
   // [bounds[i-1], bounds[i]) for 0 < i < N-1
   // [bounds[i], +infinity) for i == N-1
-  // The values in bounds array must be strictly increasing and > 0.
+  // The values in bounds array must be strictly increasing.
   //
-  // Note: only [a, b) intervals are currently supported for each bucket. If we decides
-  // to also support (a, b] intervals we should add support for these by defining a boolean
-  // value which decides what type of intervals to use.
+  // Note: only [a, b) intervals are currently supported for each bucket except the first one.
+  // If we decide to also support (a, b] intervals we should add support for these by defining
+  // a boolean value which decides what type of intervals to use.
   repeated double explicit_bounds = 7;
 }