admin: add /memory for inspection of heap usage (#4361)

Adds a /memory endpoint to the admin panel for fast inspection of Envoy::Memory's heap statistics, without needing to query stats.

Risk Level: Low
Testing: Added a test to admin_test.cc.
Docs Changes: Added a description of the option to admin.rst.
Release Notes: N/A

Signed-off-by: James Buckland <jbuckland@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ b5ba7f89a4433d283549c61a659c4e61ed564d16

envoy/admin/v2alpha/BUILD

@@ -31,3 +31,9 @@ api_proto_library_internal(
     srcs = ["metrics.proto"],
     visibility = ["//visibility:public"],
 )
+
+api_proto_library_internal(
+    name = "memory",
+    srcs = ["memory.proto"],
+    visibility = ["//visibility:public"],
+)

envoy/admin/v2alpha/memory.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+package envoy.admin.v2alpha;
+
+// [#protodoc-title: Memory]
+
+// Proto representation of the internal memory consumption of an Envoy instance. These represent
+// values extracted from an internal TCMalloc instance. For more information, see the section of the
+// docs entitled ["Generic Tcmalloc Status"](http://gperftools.github.io/gperftools/tcmalloc.html).
+message Memory {
+
+  // The number of bytes allocated by the heap for Envoy. This is an alias for
+  // `generic.current_allocated_bytes`.
+  uint64 allocated = 1;
+
+  // The number of bytes reserved by the heap but not necessarily allocated. This is an alias for
+  // `generic.heap_size`.
+  uint64 heap_size = 2;
+}