prometheus: to_str(metric_value): fix conversion type for COUNTER and DERIVE
prometheus: to_str(metric_value): fix conversion type for COUNTER and DERIVE

It looks like they got reversed.

Be consistent with metrics::impl::data_type documentation:
```
enum class data_type : uint8_t {
    COUNTER, // unsigned int 64
    GAUGE, // double
    DERIVE, // signed int 64
    ABSOLUTE, // unsigned int 64
    HISTOGRAM,
};
```

Fixes #899

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar