message: metrics: add comment explaining the namespace
diff: @@ -58,6 +58,10 @@ metrics! {
  pub(crate) html_rewrite_ooms: IntCounter,
  }
  + // The Rust prometheus library treats the namespace as the "prefix" of the metric name: a
  + // metric named `foo` with a prefix of `docsrs` will expose a metric called `docsrs_foo`.
  + //
  + // https://docs.rs/prometheus/0.9.0/prometheus/struct.Opts.html#structfield.namespace
  namespace: "docsrs",
  }
  