Additional checking for counters.

This patch adds some additional checking for counters:

* Wrap counters around at 2^32.  Most graphing software (RRDtool etc.)
  can deal with this properly.  Unfortunately wrapping at 2^64 isn't
  possible because JS has a maximum integer resolution of 2^53.

* Don't allow counters to be decremented below 0.

metrics/counter.js

@@ -1,19 +1,31 @@
 /*
 *  A simple counter object
 */
+
+/* JavaScript uses double-precision FP for all numeric types.  
+ * Perhaps someday we'll have native 64-bit integers that can safely be
+ * transported via JSON without additional code, but not today. */
+var MAX_COUNTER_VALUE = Math.pow(2, 32); // 4294967296
+
 var Counter = module.exports = function Counter() {
   this.count = 0;
   this.type = 'counter';
 }
 
 Counter.prototype.inc = function(val) {
   if (!val) { val = 1; }
-  this.count += val;
+  if (this.count === MAX_COUNTER_VALUE) {
+    this.count = 0;
+  } else {
+    this.count += val;
+  }
 }
 
 Counter.prototype.dec = function(val) {
   if (!val) { val = 1; }
-  this.count -= val;
+  if (this.count > 0) {
+    this.count -= val;
+  }
 }
 
 Counter.prototype.clear = function() {