Construct diagnostics results in the monitor thread

MariaDBMonitor diagnostics printing is unsafe as some of the read
fields are arrays. To be on the safe side, the fields are now read
in the monitor worker thread.

Since diagnostics must work even for stopped monitors, a worker task
is used. In practice, it usually runs when the monitor is sleeping.

server/core/utils.cc

@@ -1170,4 +1170,32 @@ uint8_t* set_byteN(uint8_t* ptr, uint64_t value, int bytes)
     return ptr + bytes;
 }
 
+std::string string_printf(const char* format, ...)
+{
+    /* Use 'vsnprintf' for the formatted printing. It outputs the optimal buffer length - 1. */
+    va_list args;
+    va_start(args, format);
+    int characters = vsnprintf(NULL, 0, format, args);
+    va_end(args);
+    std::string rval;
+    if (characters < 0)
+    {
+        // Encoding (programmer) error.
+        ss_dassert(!true);
+        MXS_ERROR("Could not format the string %s.", format);
+    }
+    else if (characters > 0)
+    {
+        // 'characters' does not include the \0-byte.
+        int total_size = characters + 1;
+        rval.reserve(total_size);
+        rval.resize(characters); // The final "length" of the string
+        va_start(args, format);
+        // Write directly to the string internal array, avoiding any temporary arrays.
+        vsnprintf(&rval[0], total_size, format, args);
+        va_end(args);
+    }
+    return rval;
+}
+
 }