v0.9.0

• prometheus/metrics (breaking): the reading that says which build wrote the page is named for the tool, as Prometheus's own tools name theirs: sas-exporter gives sas_exporter_build_info. Options.Namespace is gone, so a tool drops it; Options.BuildInfo names the reading outright, to keep a name a dashboard already uses
• prometheus/metricstest: a saved page leaves out the build's reading whatever the tool is called