fuzz: add bounds to statsh flush interval (#8043)

Add PGV bounds to the stats flush interval (greater than 1ms and less than 5000ms) to prevent Envoy from hanging from too small of a flush time.

Risk Level: Low
Testing: Corpus Entry added
Fixes OSS-Fuzz issue
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=16300

Signed-off-by: Asra Ali <asraa@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0a3fc6a091a768ba868ff4dd94cf526af4ffc0ff

envoy/config/bootstrap/v2/bootstrap.proto

@@ -99,7 +99,14 @@ message Bootstrap {
   // performance reasons Envoy latches counters and only flushes counters and
   // gauges at a periodic interval. If not specified the default is 5000ms (5
   // seconds).
-  google.protobuf.Duration stats_flush_interval = 7 [(gogoproto.stdduration) = true];
+  // Duration must be at least 1ms and at most 5 min.
+  google.protobuf.Duration stats_flush_interval = 7 [
+    (validate.rules).duration = {
+      lt: {seconds: 300},
+      gte: {nanos: 1000000}
+    },
+    (gogoproto.stdduration) = true
+  ];
 
   // Optional watchdog configuration.
   Watchdog watchdog = 8;