Add prometheus metrics for Qualifiers

[tomcoldrick-ct]

In #12 it was mentioned that we might want to add metrics for the qualifiers passed through the asset server, but we didn't want to block the PR on that. This should probably be in separate metrics for Fetch/Push.

[Qinusty]

#23 adds the resource_type qualifier.

[jjardon]

@tomcoldrick-ct I think this can be closed now?