Add subgraph push option to specify endpoint (#130)

docs/source/subgraphs.md

@@ -52,14 +52,23 @@ a `.graphql` SDL file, you can pass the path to that file as the `--schema` flag
 (or `-s` for short):
 
 ```bash
-rover subgraph push my-graph --schema ./accounts/schema.graphql --service-name accounts
+rover subgraph push my-graph\
+  --schema ./accounts/schema.graphql\
+  --service-name accounts\
+  --routing-url https://my-running-service.com/api
 ```
 
 this will push the schema at ./accounts/schema.graphql to the default (`current`) variant
 of `my-graph`. You can specify a different variant by adding the variant name
 after the graph name like `my-graph@variant-name`. The `--service-name` flag
 just tells the registry which subgraph you're pushing changes to.
 
+The `--routing-url` is used by a gateway running in [managed federation mode]
+(https://www.apollographql.com/docs/federation/managed-federation/overview/). If
+You're running a service which hasn't been deployed yet or isn't using managed
+federation, you may pass a placeholder url or leave the flag empty
+(i.e. `--routing-url=""`).
+
 If you're not using an SDL file to build your schema, you'll likely want to
 introspect your subgraph to push it. Rover supports accepting `stdin` for the
 schema flag just for cases like this. You can use the `stdout` of another
@@ -70,7 +79,10 @@ any command with a `--schema` flag. To do this, pass `-` as the value for the
 > TODO: check this API whenever the introspect command lands
 
 ```sh
-rover subgraph introspect http://localhost:4001 | rover subgraph push my-graph@dev --schema - --service-name accounts
+rover subgraph introspect http://localhost:4001\
+  | rover subgraph push my-graph@dev\
+  --schema - --service-name accounts\
+  --routing-url https://my-running-service.com/api
 ```
 
 > For more on how `stdin` works, see [Essential Concepts](./essentials#using-stdin).
@@ -89,10 +101,15 @@ subgraph's service name and schema to check changes for using the same
 
 ```bash
 # using a schema file
-rover subgraph check my-graph --schema ./accounts/schema.graphql --service-name accounts
+rover subgraph check my-graph\
+  --schema ./accounts/schema.graphql\
+  --service-name accounts
 
 # using introspection
-rover subgraph introspect http://localhost:4000 | rover subgraph check my-graph --schema - --service-name accounts
+rover subgraph introspect http://localhost:4000\
+  | rover subgraph check my-graph\
+  --schema -\
+  --service-name accounts
 ```
 
 > TODO: check after introspection lands

src/command/subgraph/push.rs

@@ -32,6 +32,12 @@ pub struct Push {
     #[structopt(long)]
     #[serde(skip_serializing)]
     service_name: String,
+
+    /// Url of a running service that a gateway can route operations to
+    /// (often a deployed service). May be left empty ("") or a placeholder url
+    /// if not running a gateway in managed federation mode
+    #[structopt(long)]
+    routing_url: String,
 }
 
 impl Push {
@@ -58,7 +64,7 @@ impl Push {
                     hash: None,
                 },
                 revision: "".to_string(),
-                url: "".to_string(),
+                url: self.routing_url.clone(),
             },
             &client,
         )