MB-45090: Initialize couchstore write-ampl stats to 0

Change-Id: I4006c990b503e868cad1dc461c6c6c2404942013
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/149311
Reviewed-by: Trond Norbye <trond.norbye@couchbase.com>
Tested-by: Paolo Cocchi <paolo.cocchi@couchbase.com>

engines/ep/src/ep_engine.cc

@@ -2665,7 +2665,7 @@ void EventuallyPersistentEngine::doEngineStatsCouchDB(const BucketStatCollector&
         auto printWriteAmpStat = [this, &collector, docBytes = value](
                 const char* writeBytesStat,
                 const char* writeAmpStat) {
-          double writeAmp = std::numeric_limits<double>::infinity();
+          double writeAmp = 0;
           size_t bytesWritten;
           if (docBytes &&
               kvBucket->getKVStoreStat(writeBytesStat,