Prefer G_GINT64_FORMAT to PRId64.
Prefer G_GINT64_FORMAT to PRId64.

We don't need to include inttypes.h just to use PRId64 when GLib already
provides an equivalent macro. Also, we use the G_*_FORMAT macros in
other places in the metrics system, but this was the only remaining
use of PRId64.

[endlessm/eos-sdk#3254]

endlessm/eos-metrics,endlessm/eos-metrics,endlessm/eos-metrics,endlessm/eos-metrics,endlessm/eos-metrics,endlessm/eos-metrics