What single, most indicative monitoring metric should be tracked by an integrating client to assess the real-time health and potential overload of the OTP service?

The single, most indicative monitoring metric an integrating client should track to assess the real-time health and potential overload of an OTP service is the 99th Percentile Response Time. Response time, also known as latency, is the total duration an integrating client waits from sending a request to the OTP service until receiving a complete response. It directly measures how quickly the OTP service processes and responds to requests. The 99th percentile (P99) specifically indicates the maximum response time for 99% of all requests. This means that only 1% of requests took longer than the P99 value. This particular percentile is crucial because it captures the experience of nearly all users, including those who encounter the slower, 'tail-end' requests, which are often the first to be affected by performance degradation or an approaching overload state. A consistently low P99 response time signifies a healthy OTP service that is processing requests efficiently. Conversely, a rising P99 response time indicates that the service is taking longer to respond, suggesting potential performance bottlenecks, resource saturation, or an increasing queue of pending requests within the OTP service. Such an increase directly signals that the OTP service is struggling to cope with the current request volume, thus indicating real-time degradation in health and imminent or actual overload. Unlike simple average response time, which can hide significant delays experienced by a subset of requests, P99 accurately reflects the worst-case experience for the vast majority of requests, making it a highly sensitive and reliable indicator for both current health and the onset of overload from the integrating client's perspective.