Healthcheck filter: compute response based on upstream cluster health (#417)

Signed-off-by: Brian Pane <bpane@pinterest.com>

STYLE.md

@@ -67,3 +67,5 @@ In addition, the following conventions should be followed:
   ```
 
   This is more efficient, extendable and self-describing.
+
+* To represent percentage values, use the Percent message type defined in [api/base.proto](api/base.proto).

api/base.proto

@@ -35,6 +35,11 @@ message Locality {
   string sub_zone = 3;
 }
 
+// Identifies a percentage, in the range [0.0, 100.0].
+message Percent {
+  double value = 1 [(validate.rules).double = {gte: 0, lte: 100}];
+}
+
 // Identifies a specific Envoy instance. The node identifier is presented to the
 // management server, which may use this identifier to distinguish per Envoy
 // configuration for serving.

api/filter/http/BUILD

@@ -37,6 +37,9 @@ api_proto_library(
 api_proto_library(
     name = "health_check",
     srcs = ["health_check.proto"],
+    deps = [
+        "//api:base",
+    ],
 )
 
 api_proto_library(

api/filter/http/health_check.proto

@@ -5,6 +5,7 @@ package envoy.api.v2.filter.http;
 import "google/protobuf/duration.proto";
 import "google/protobuf/wrappers.proto";
 
+import "api/base.proto";
 import "validate/validate.proto";
 
 // [#protodoc-title: Health check]
@@ -21,4 +22,10 @@ message HealthCheck {
   // If operating in pass through mode, the amount of time in milliseconds
   // that the filter should cache the upstream response.
   google.protobuf.Duration cache_time = 3;
+
+  // [#not-implemented-hide:]
+  // If operating in non-pass-through mode, specifies a set of upstream cluster
+  // names and the minimum percentage of servers in each of those clusters that
+  //  must be healthy in order for the filter to return a 200.
+  map<string, Percent> cluster_min_healthy_percentages = 4;
 }