What is "signpost_reporter"?
Signposts is a developer feature created by Apple to help developers diagnose performance problems in applications. 

Essentially developer can create a “signpost” marking the beginning and end of performance critical operations. The signpost system then automatically times these intervals and makes them available to profiling tools on the local machine.

The signpost_reporter program is used to report these timings back to Apple. I.e. it allows Apple to monitor performance on non-developer machines by sending the telemetry data from the signpost feature to Apple. For example Apple might be interested in knowing how often customers experience problems with a specific feature being too slow.

Without seeing the actual log entry, it’s not possible to say why you got a log entry for it. However rest assured that your system works just as well with and without the signpost_reporter.