Add List method to gRPC Health service (#143)

* Add List method to Health service

* Apply suggestions from code review

Co-authored-by: Benjamin Krämer <[email protected]>

* Update List method with feedback from the proposal review process

* Apply suggestions from code review

Signed-off-by: Marcos Huck <[email protected]>

* Soften warning about too many services

Signed-off-by: Marcos Huck <[email protected]>

---------

Signed-off-by: Marcos Huck <[email protected]>
Co-authored-by: Benjamin Krämer <[email protected]>

Author: marcoshuck

grpc/health/v1/health.proto

@@ -40,6 +40,13 @@ message HealthCheckResponse {
   ServingStatus status = 1;
 }
 
+message HealthListRequest {}
+
+message HealthListResponse {
+  // statuses contains all the services and their respective status.
+  map<string, HealthCheckResponse> statuses = 1;
+}
+
 // Health is gRPC's mechanism for checking whether a server is able to handle
 // RPCs. Its semantics are documented in
 // https://github.com/grpc/grpc/blob/master/doc/health-checking.md.
@@ -51,10 +58,21 @@ service Health {
   //
   // Clients should set a deadline when calling Check, and can declare the
   // server unhealthy if they do not receive a timely response.
-  //
-  // Check implementations should be idempotent and side effect free.
   rpc Check(HealthCheckRequest) returns (HealthCheckResponse);
 
+  // List provides a non-atomic snapshot of the health of all the available
+  // services.
+  //
+  // The server may respond with a RESOURCE_EXHAUSTED error if too many services
+  // exist.
+  //
+  // Clients should set a deadline when calling List, and can declare the server
+  // unhealthy if they do not receive a timely response.
+  //
+  // Clients should keep in mind that the list of health services exposed by an
+  // application can change over the lifetime of the process.
+  rpc List(HealthListRequest) returns (HealthListResponse);
+
   // Performs a watch for the serving status of the requested service.
   // The server will immediately send back a message indicating the current
   // serving status.  It will then subsequently send a new message whenever