add resource monitor framework (#3848)

Add an extensible resource monitor framework for monitoring resource "pressures" (usage/limit). This will be used by the overload manager to implement downstream circuit breaking (issue #373 - see design doc linked from there).

Risk Level: low (not yet used in envoy main)

Signed-off-by: Elisha Ziskind <eziskind@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0e71582b338b97d5667be7deaff831a062a96d1f

envoy/config/resource_monitor/fixed_heap/v2alpha/BUILD

@@ -0,0 +1,8 @@
+load("//bazel:api_build_system.bzl", "api_proto_library_internal")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_library_internal(
+    name = "fixed_heap",
+    srcs = ["fixed_heap.proto"],
+)

envoy/config/resource_monitor/fixed_heap/v2alpha/fixed_heap.proto

@@ -0,0 +1,10 @@
+syntax = "proto3";
+
+package envoy.config.resource_monitor.fixed_heap.v2alpha;
+option go_package = "v2alpha";
+
+message FixedHeapConfig {
+  // Limit of the Envoy process heap size. This is used to calculate heap memory pressure which
+  // is defined as (current heap size)/max_heap_size_bytes.
+  uint64 max_heap_size_bytes = 1;
+}