io_tester: disambiguate sstring streaming to YAML::Emitter

Newer yaml-cpp (>= 0.8.0) adds an
  operator<<(Emitter&, const std::string_view&)
overload alongside the existing const std::string& one. Since
seastar::sstring is implicitly convertible to both std::string and
std::string_view, streaming an sstring directly into a YAML::Emitter
became ambiguous and broke the build (seen on the Alpine CI image):

  error: ambiguous overload for 'operator<<'
    (operand types are 'YAML::Emitter' and 'seastar::sstring')

Explicitly convert to std::string at the two affected call sites. The
std::string overload has always existed, so this remains correct on
older yaml-cpp versions as well.

Author: travisdowns

apps/io_tester/io_tester.cc

@@ -648,7 +648,7 @@ class io_class_data : public class_data {
 
     void emit_one_metrics(YAML::Emitter& out, sstring m_name, bool check_class_metrics = true) {
         if (auto v = get_one_metrics(m_name, check_class_metrics); v.has_value()) {
-            out << YAML::Key << m_name << YAML::Value << *v;
+            out << YAML::Key << std::string(m_name) << YAML::Value << *v;
         }
     }
 
@@ -1206,7 +1206,7 @@ class context {
     future<> emit_results(YAML::Emitter& out) {
         return _finished.wait(_cl.size()).then([this, &out] {
             for (auto& cl: _cl) {
-                out << YAML::Key << cl->name();
+                out << YAML::Key << std::string(cl->name());
                 out << YAML::BeginMap;
                 cl->emit_results(out);
                 out << YAML::EndMap;