Add note about request tracing with AWS LB (#279)

Author: sosedoff

README.md

@@ -570,7 +570,7 @@ Request Queueing is defined as the time it takes for a request to reach your app
 
 As this metric starts before `prometheus_exporter` can handle the request, you must add a specific HTTP header as early in your infrastructure as possible (we recommend your load balancer or reverse proxy).
 
-The Amazon Application Load Balancer [request tracing header](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-request-tracing.html) is natively supported. If you are using another upstream entrypoint, you may configure your HTTP server / load balancer to add a header `X-Request-Start: t=<MSEC>` when passing the request upstream. For more information, please consult your software manual.
+The Amazon Application Load Balancer [request tracing header](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-request-tracing.html) is natively supported. If you are using another upstream entrypoint, you may configure your HTTP server / load balancer to add a header `X-Request-Start: t=<MSEC>` when passing the request upstream. Please keep in mind request time start is reported as epoch time (in seconds) and lacks precision, which may introduce additional latency in reported metrics. For more information, please consult your software manual.
 
 Hint: we aim to be API-compatible with the big APM solutions, so if you've got requests queueing time configured for them, it should be expected to also work with `prometheus_exporter`.