Encode the required fail-fast behavior as tests ahead of the
implementation: a config value that is SET but unparseable or invalid
must abort startup (defaults apply only to OMITTED keys), unknown
top-level keys and unknown metrics subkeys must abort naming the key,
a malformed config file at a standard location must abort instead of
being skipped with a warning, and state_dir must be creatable and
writable at startup. Mechanically extracts newFromSmartConfig from
config.New so the construction path is testable without fx; current
lenient behavior is unchanged, so the new enforcement tests fail.
sneak
f19da2c02c