Add ToString for time_points

Adds ToString helper functions for aos's time_point classes that match
the format that LogReader expects for start/end_time.

Change-Id: Ied8635da70ceabb82328ea47b34f878084663bdd
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/time/time.cc b/aos/time/time.cc
index db71379..e75fe45 100644
--- a/aos/time/time.cc
+++ b/aos/time/time.cc
@@ -6,6 +6,7 @@
 #include <cstring>
 #include <ctime>
 #include <iomanip>
+#include <sstream>
 
 #ifdef __linux__
 
@@ -79,6 +80,18 @@
   return stream;
 }
 
+std::string ToString(const aos::monotonic_clock::time_point &now) {
+  std::ostringstream stream;
+  stream << now;
+  return stream.str();
+}
+
+std::string ToString(const aos::realtime_clock::time_point &now) {
+  std::ostringstream stream;
+  stream << now;
+  return stream.str();
+}
+
 #ifdef __linux__
 std::optional<monotonic_clock::time_point> monotonic_clock::FromString(
     const std::string_view now) {