common: use fmt::format for stderr logging

Reduce the number of syscalls to improve performance.

Also reduce the probability of https://tracker.ceph.com/issues/49551,
since conmon is less likely to read partial log line.

Test shows that `fmt::print(cerr, ...)` is as performant as calling
`writev` directly. And it is portable and safe in handling short write.

fmt::print(stderr, ...) is not used because it can throw exception.

Fixes: https://tracker.ceph.com/issues/53682
Signed-off-by: 胡玮文 <huww98@outlook.com>

src/log/Log.cc

@@ -26,6 +26,7 @@
 #include <set>
 
 #include <fmt/format.h>
+#include <fmt/ostream.h>
 
 #define MAX_LOG_BUF 65536
 
@@ -312,13 +313,13 @@ void Log::_flush(EntryVector& t, bool crash)
         syslog(LOG_USER|LOG_INFO, "%s", pos);
       }
 
-      if (do_stderr) {
-        std::cerr << m_log_stderr_prefix << std::string_view(pos, used) << std::endl;
-      }
-
       /* now add newline */
       pos[used++] = '\n';
 
+      if (do_stderr) {
+        fmt::print(std::cerr, "{}{}", m_log_stderr_prefix, std::string_view(pos, used));
+      }
+
       if (do_fd) {
         m_log_buf.resize(cur + used);
       } else {