Add dump function to nearby connections.

Sample output:
```
Time: 2022-06-23T17:24:14.3815934+00:00
Windows version: 10.0 build 19042
NearbyShare version: 1.2.3.4
Nearby Share Settings
  Device name: Deling's PC
  Visibility: 1
  Enabled: 1
  ShowNotification: 0
  DataUsage: 0
  Allowed contacts:
Nearby Sharing Service state
  IsScanning: 0
  IsConnecting: 0
  IsTransferring: 0
  IsSendingFile: 0
  IsReceivingFile: 0
Nearby Connections State
  Client ID: -5841239073452700193
  Local Endpoint ID: IWV0
  High Vis Mode: 0
  Is Advertising: 0
  Advertising Service ID:
  Is Discovering: 0
  Discovery Service ID:
  Connections:
  Discovered endpoint IDs:
```
PiperOrigin-RevId: 458066027

connections/core.cc

@@ -168,6 +168,10 @@ void Core::StopAllEndpoints(ResultCallback callback) {
   router_->StopAllEndpoints(&client_, callback);
 }
 
+std::string Core::Dump() {
+  return client_.Dump();
+}
+
 }  // namespace connections
 }  // namespace nearby
 }  // namespace location

connections/core.h

@@ -238,6 +238,8 @@ class Core {
   // Gets the local endpoint generated by Nearby Connections.
   std::string GetLocalEndpointId() { return client_.GetLocalEndpointId(); }
 
+  std::string Dump();
+
  private:
   ClientProxy client_;
   ServiceControllerRouter* router_ = nullptr;

connections/implementation/BUILD

@@ -148,6 +148,7 @@ cc_library(
         "@com_google_absl//absl/functional:bind_front",
         "@com_google_absl//absl/memory",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/strings:str_format",
         "@com_google_absl//absl/time",
         "@com_google_absl//absl/types:span",
     ],
@@ -232,7 +233,6 @@ cc_test(
         ":ukey2",
         "//connections:core_types",
         "//connections/implementation/mediums",
-        "//connections/implementation/mediums:utils",
         "//connections/implementation/proto:offline_wire_formats_cc_proto",
         "//internal/platform:base",
         "//internal/platform:comm",
@@ -244,6 +244,7 @@ cc_test(
         "@com_github_protobuf_matchers//protobuf-matchers",
         "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/strings:str_format",
         "@com_google_absl//absl/synchronization",
         "@com_google_absl//absl/time",
         "@com_google_absl//absl/types:span",

connections/implementation/client_proxy.cc

@@ -17,6 +17,7 @@
 #include <cstdlib>
 #include <functional>
 #include <limits>
+#include <sstream>
 #include <string>
 #include <utility>
 
@@ -24,6 +25,7 @@
 #include "absl/container/flat_hash_set.h"
 #include "absl/strings/escaping.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
 #include "internal/platform/error_code_recorder.h"
 #include "internal/platform/feature_flags.h"
 #include "internal/platform/logging.h"
@@ -762,6 +764,35 @@ std::string ClientProxy::ToString(PayloadProgressInfo::Status status) const {
   }
 }
 
+std::string ClientProxy::Dump() {
+  std::stringstream sstream;
+  sstream << "Nearby Connections State" << std::endl;
+  sstream << "  Client ID: " << GetClientId() << std::endl;
+  sstream << "  Local Endpoint ID: " << GetLocalEndpointId() << std::endl;
+  sstream << "  High Visibility Mode: " << high_vis_mode_ << std::endl;
+  sstream << "  Is Advertising: " << IsAdvertising() << std::endl;
+  sstream << "  Advertising Service ID: " << GetAdvertisingServiceId()
+          << std::endl;
+  // TODO(deling): AdvertisingOptions
+  sstream << "  Is Discovering: " << IsDiscovering() << std::endl;
+  sstream << "  Discovery Service ID: " << GetDiscoveryServiceId() << std::endl;
+  // TODO(deling): DiscoveryOptions
+
+  sstream << "  Connections: " << std::endl;
+  for (auto it = connections_.begin(); it != connections_.end(); ++it) {
+    // TODO(deling): write Connection.ToString()
+    sstream << "    " << it->first << " : " << it->second.connection_token
+            << std::endl;
+  }
+
+  sstream << "  Discovered endpoint IDs: " << std::endl;
+  for (auto it = discovered_endpoint_ids_.begin();
+       it != discovered_endpoint_ids_.end(); ++it) {
+    sstream << "    " << *it << std::endl;
+  }
+
+  return sstream.str();
+}
 }  // namespace connections
 }  // namespace nearby
 }  // namespace location

connections/implementation/client_proxy.h

@@ -188,6 +188,8 @@ class ClientProxy final {
   // rotates.
   void ExitHighVisibilityMode();
 
+  std::string Dump();
+
  private:
   struct Connection {
     // Status: may be either:

connections/implementation/client_proxy_test.cc

@@ -14,12 +14,14 @@
 
 #include "connections/implementation/client_proxy.h"
 
+#include <cstdio>
 #include <string>
 
 #include "gmock/gmock.h"
 #include "protobuf-matchers/protocol-buffer-matchers.h"
 #include "gtest/gtest.h"
 #include "absl/container/flat_hash_set.h"
+#include "absl/strings/str_format.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 #include "absl/types/span.h"
@@ -362,6 +364,23 @@ TEST_F(ClientProxyTest, ClientIdIsUnique) {
   EXPECT_NE(client1_.GetClientId(), client2_.GetClientId());
 }
 
+TEST_F(ClientProxyTest, DumpString) {
+  std::string expect = absl::StrFormat(
+      "Nearby Connections State\n"
+      "  Client ID: %d\n"
+      "  Local Endpoint ID: %s\n"
+      "  High Visibility Mode: 0\n"
+      "  Is Advertising: 0\n"
+      "  Advertising Service ID: \n"
+      "  Is Discovering: 0\n"
+      "  Discovery Service ID: \n"
+      "  Connections: \n"
+      "  Discovered endpoint IDs: \n",
+      client1_.GetClientId(), client1_.GetLocalEndpointId());
+  std::string dump = client1_.Dump();
+  EXPECT_EQ(dump, expect);
+}
+
 TEST_F(ClientProxyTest, GeneratedEndpointIdIsUnique) {
   EXPECT_NE(client1_.GetLocalEndpointId(), client2_.GetLocalEndpointId());
 }