These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.05 s (backtrack: 0/20).

[ebuild  N     ] www-servers/civetweb-1.16::gentoo  USE="cxx server ssl" 11812 KiB
[ebuild  N     ] dev-cpp/prometheus-cpp-1.2.4::gentoo  USE="zlib -test" 73 KiB
[ebuild  N     ] app-metrics/portage-exporter-0.1.0::gentoo  15 KiB

Total: 3 packages (3 new), Size of downloads: 11899 KiB

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by dev-cpp/prometheus-cpp-1.2.4::gentoo
# required by app-metrics/portage-exporter-0.1.0::gentoo
# required by =app-metrics/portage-exporter-0.1.0 (argument)
>=www-servers/civetweb-1.16 cxx