http_client: http_client_req() returns incorrect number of bytes sent

[ewalt1]

The http_client_req() function is supposed to return the number of bytes sent to the server:
https://github.com/zephyrproject-rtos/zephyr/blob/main/include/net/http_client.h#L327

However, there is a bug in the implementation such that it generally returns something much larger. Each line of the headers is sent via one call to http_send_data(), the return value of which is added to the total_sent summation variable.
https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/net/lib/http/http_client.c#L529

However, http_send_data() doesn't typically return the number of bytes that were added in that particular invocation. Instead it copies the values into a local 192 byte buffer and returns the current position within that buffer.

For example, if I wanted to send:

        PATCH /v1/123456789abcd HTTP/1.1
        Host: xxxxxxxxxxxxxxxxxx.uc.r.appspot.com
        Content-Length: 34

Then the http_client sends 33+43+22==98 bytes of headers plus my 34 bytes of payload for a total of 132 bytes.

However the calls to http_send_data() will return 33, (33 + 43), and (33 + 43 + 22) respectively. This leads to total_sent being set to 207 by the time the headers have been sent, which causes an incorrect return value of 207+34==241 from http_client_req().

[rlubos]

@ewalt1 Please check #43916 where I proposed a fix for the problem.