Initialize start time for server (#2220)

* Initialize start time for server

By initializing start_time_ for server, I hope to measure the time taken to process a request at the end maybe in the set_logger callback and print it.

I only see current usage in client with server retaining the inital min value

* Add test to verify start time is initialized

* Address review comments

* run clang format

httplib.h

@@ -8264,6 +8264,7 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
   if (!line_reader.getline()) { return false; }
 
   Request req;
+  req.start_time_ = std::chrono::steady_clock::now();
 
   Response res;
   res.version = "HTTP/1.1";

test/test.cc

@@ -3200,6 +3200,11 @@ protected:
              [&](const Request & /*req*/, Response &res) {
                res.set_content("abcdefg", "text/plain");
              })
+        .Get("/test-start-time",
+             [&](const Request &req, Response &res) {
+               EXPECT_NE(req.start_time_,
+                         std::chrono::steady_clock::time_point::min());
+             })
         .Get("/with-range-customized-response",
              [&](const Request & /*req*/, Response &res) {
                res.status = StatusCode::BadRequest_400;
@@ -5800,6 +5805,8 @@ TEST_F(ServerTest, TooManyRedirect) {
   EXPECT_EQ(Error::ExceedRedirectCount, res.error());
 }
 
+TEST_F(ServerTest, StartTime) { auto res = cli_.Get("/test-start-time"); }
+
 #ifdef CPPHTTPLIB_ZLIB_SUPPORT
 TEST_F(ServerTest, Gzip) {
   Headers headers;