This patch allows static decomposition configuration to easily access
the record member as follows:

```
	...
        { "src":"netdev_list[rx_bytes]" },
	...
```

If "fill" was correctly specified, `cfg_col->fill` will have a value.
Otherwise, `cfg-col->fill` will be `NULL`. We should use `cfg_col->fill`
to determine if "fill" was present.

The "fill" string value was handled incorrectly. This patch fixes the
bug.

Test sampler access raw metric value with `mval->v_u64` and increment
that value by 1. This works for integer values. When `mval` contains
float or double, `mval->v_u64` became big number due to IEEE 754
binary32 or binary64 format. As such, `test_sampler` should use
`ldms_mval_as_u64()` to coerce the `mval` to u64 properly.