[clangd] Record remote index usage

[kadircet]

Summary

This is a gauage metric that sets particular remote-index instances as
used. It should enable accumulation of multiple streams to see number of clangd
processes making use of remote index, broken down by remote index address.

clang-tools-extra/clangd/tool/ClangdMain.cpp

 const char TestScheme::TestDir[] = "C:\\clangd-test";
 #else
 const char TestScheme::TestDir[] = "/clangd-test";
 #endif
 
 std::unique_ptr<SymbolIndex>
 loadExternalIndex(const Config::ExternalIndexSpec &External,
                   AsyncTaskRunner *Tasks) {
+  static const trace::Metric RemoteIndexUsed("used_remote_index",
+                                             trace::Metric::Value, "address");
   switch (External.Kind) {
   case Config::ExternalIndexSpec::None:
     break;
   case Config::ExternalIndexSpec::Server:
+    RemoteIndexUsed.record(1, External.Location);
     log("Associating {0} with remote index at {1}.", External.MountPoint,
         External.Location);
     return remote::getClient(External.Location, External.MountPoint);
   case Config::ExternalIndexSpec::File:
     log("Associating {0} with monolithic index at {1}.", External.MountPoint,
         External.Location);
     auto NewIndex = std::make_unique<SwapIndex>(std::make_unique<MemIndex>());
     auto IndexLoadTask = [File = External.Location,