Metric names containing spaces are not valid

[08d2]

Code like e.g. this suggests that this crate permits metric names containing spaces. The data model asserts that

The metric name. . . may contain ASCII letters and digits, as well as underscores and colons. It must match the regex [a-zA-Z_:][a-zA-Z0-9_:]*.

Names that don't meet these criteria should be rejected.

[mxinden]

See also https://github.com/prometheus/client_rust/issues/52.