examples/cpp/route_guide add missing command line parsing (grpc#37857)

The gRPC basic turorial says to call the following command to run the server `./route_guide_server --db_path=path/to/route_guide_db.json`

But the server and client do not parse command line arguments and they always use default value `route_guide_db.json`. Thus the example only works if we run it from `examples/cpp/route_guide` dir. Otherwise we get the following error: `helper.cc:49] Failed to open route_guide_db.json
Aborted (core dumped)`

Add
`absl::ParseCommandLine(argc, argv);`
to both server and client `main` similar to `route_guide_callback_server.cc`

Closes grpc#37857

PiperOrigin-RevId: 709154601

Author: RomantsovS

examples/cpp/route_guide/BUILD

@@ -39,6 +39,7 @@ cc_binary(
         ":route_guide_helper",
         "//:grpc++",
         "//examples/protos:route_guide",
+        "@com_google_absl//absl/flags:parse",
     ],
 )
 
@@ -53,6 +54,7 @@ cc_binary(
         ":route_guide_helper",
         "//:grpc++",
         "//examples/protos:route_guide",
+        "@com_google_absl//absl/flags:parse",
     ],
 )
 

examples/cpp/route_guide/Makefile

@@ -29,6 +29,7 @@
 #include <string>
 #include <thread>
 
+#include "absl/flags/parse.h"
 #include "helper.h"
 #ifdef BAZEL_BUILD
 #include "examples/protos/route_guide.grpc.pb.h"
@@ -216,6 +217,7 @@ class RouteGuideClient {
 };
 
 int main(int argc, char** argv) {
+  absl::ParseCommandLine(argc, argv);
   // Expect only arg: --db_path=path/to/route_guide_db.json.
   std::string db = routeguide::GetDbFileContent(argc, argv);
   RouteGuideClient guide(

examples/cpp/route_guide/route_guide_client.cc

@@ -29,6 +29,7 @@
 #include <memory>
 #include <string>
 
+#include "absl/flags/parse.h"
 #include "helper.h"
 #ifdef BAZEL_BUILD
 #include "examples/protos/route_guide.grpc.pb.h"
@@ -182,6 +183,7 @@ void RunServer(const std::string& db_path) {
 }
 
 int main(int argc, char** argv) {
+  absl::ParseCommandLine(argc, argv);
   // Expect only arg: --db_path=path/to/route_guide_db.json.
   std::string db = routeguide::GetDbFileContent(argc, argv);
   RunServer(db);