gnu: Prometheus client: Update to 0.9.2.
* gnu/packages/syncthing.scm
(go-github-com-client-golang-prometheus-promhttp,
go-github-com-client-golang-prometheus): Update to 0.9.2.
(go-github-com-client-golang-prometheus-promhttp)
[propagated-inputs]: Add go-github-com-prometheus-common-model and
go-github-com-prometheus-common-internal-bitbucket-org-ww-goautoneg.
(go-github-com-client-golang-prometheus)[propagated-inputs]: Remove
go-github-com-golang-protobuf-proto and
go-github-com-client-golang-prometheus-promhttp. Add
go-github-com-prometheus-common-model and
go-github-com-prometheus-common-internal-bitbucket-org-ww-goautoneg.
(go-github-com-prometheus-union): Delete variable.