CacheVC::openWriteCloseDir: fix metric array indexing (#10682)

The fragment_document_count is an array of size 3. Our logic for
accessing it was supposed to clamp indexing into it between values
0 through 2, but accidentally allowed indexing outside of that range.
This resulted in an out of bounds index into random memory. This fixes
the logic to properly clamp to the appropriate values.

For reference here's the old logic before our recent Metrics change:
https://github.com/apache/trafficserver/pull/10175/files#diff-6ac65bcc29d9196567a0bbff3c7aa6705ebb7a6041f4a5706b6b475202d4934dL1200

src/iocore/cache/CacheWrite.cc

@@ -1048,8 +1048,8 @@ CacheVC::openWriteCloseDir(int /* event ATS_UNUSED */, Event * /* e ATS_UNUSED *
   if ((closed == 1) && (total_len > 0 || f.allow_empty_doc)) {
     DDbg(dbg_ctl_cache_stats, "Fragment = %d", fragment);
 
-    Metrics::increment(cache_rsb.fragment_document_count[std::max(fragment, 2)]);
-    Metrics::increment(vol->cache_vol->vol_rsb.fragment_document_count[std::max(fragment, 2)]);
+    Metrics::increment(cache_rsb.fragment_document_count[std::clamp(fragment, 0, 2)]);
+    Metrics::increment(vol->cache_vol->vol_rsb.fragment_document_count[std::clamp(fragment, 0, 2)]);
   }
   if (f.close_complete) {
     recursive++;