memory: Add additional tcmalloc stats (#4983)

Signed-off-by: Tony Allen <tallen@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ 9c0ff96ead873153b897bd22e993938a68066ef0

envoy/admin/v2alpha/memory.proto

@@ -16,4 +16,18 @@ message Memory {
   // The number of bytes reserved by the heap but not necessarily allocated. This is an alias for
   // `generic.heap_size`.
   uint64 heap_size = 2;
+
+  // The number of bytes in free, unmapped pages in the page heap. These bytes always count towards
+  // virtual memory usage, and depending on the OS, typically do not count towards physical memory
+  // usage. This is an alias for `tcmalloc.pageheap_unmapped_bytes`.
+  uint64 pageheap_unmapped = 3;
+
+  // The number of bytes in free, mapped pages in the page heap. These bytes always count towards
+  // virtual memory usage, and unless the underlying memory is swapped out by the OS, they also
+  // count towards physical memory usage. This is an alias for `tcmalloc.pageheap_free_bytes`.
+  uint64 pageheap_free = 4;
+
+  // The amount of memory used by the TCMalloc thread caches (for small objects). This is an alias
+  // for `tcmalloc.current_total_thread_cache_bytes`.
+  uint64 total_thread_cache = 5;
 }