Add timeout for RTT monitor "hello" operations

Currently the RTT monitor uses a context without timeout to run the "hello" operation (see here). As a result, it's possible for RTT monitor "hello" operations to hang indefinitely if there is a network interruption, preventing the monitor from recording any RTT samples. The motivation for this ticket comes from troubleshooting GODRIVER-2438.

Add a timeout to the RTT monitor "hello" operation to prevent network issues from causing the RTT monitor to hang.