fix(subscriber): use saturating arithmetic for attribute updates (#234)

Currently, an issue exists where a subtraction attribute update for a
resource or async op attribute may overflow, if a previous add update
was dropped due to event buffer capacity. This may result in the
aggregator task panicking. See
#180 (comment)
for details.

This branch changes all resource updates to use saturating arithmetic.
In practice, I think it's _much_ more likely for a subtract to overflow
due to missed adds than it is for an add to overflow due to missed subs,
but I made the addition saturating as well, just in case.

We should probably also increase the default event buffer capacity,
since it seems like lots of people are hitting problems with this...

console-subscriber/src/aggregator/mod.rs

@@ -1074,9 +1074,9 @@ fn update_attribute(attribute: &mut Attribute, update: &AttributeUpdate) {
         (Some(DebugVal(v)), Some(DebugVal(upd))) => *v = upd,
 
         (Some(U64Val(v)), Some(U64Val(upd))) => match update.op {
-            Some(AttributeUpdateOp::Add) => *v += upd,
+            Some(AttributeUpdateOp::Add) => *v = v.saturating_add(upd),
 
-            Some(AttributeUpdateOp::Sub) => *v -= upd,
+            Some(AttributeUpdateOp::Sub) => *v = v.saturating_sub(upd),
 
             Some(AttributeUpdateOp::Override) => *v = upd,
 
@@ -1087,9 +1087,9 @@ fn update_attribute(attribute: &mut Attribute, update: &AttributeUpdate) {
         },
 
         (Some(I64Val(v)), Some(I64Val(upd))) => match update.op {
-            Some(AttributeUpdateOp::Add) => *v += upd,
+            Some(AttributeUpdateOp::Add) => *v = v.saturating_add(upd),
 
-            Some(AttributeUpdateOp::Sub) => *v -= upd,
+            Some(AttributeUpdateOp::Sub) => *v = v.saturating_sub(upd),
 
             Some(AttributeUpdateOp::Override) => *v = upd,