VM attempts to parse IP addr as float

### Summary
Certain regex patterns when used to capture an IP address are causing mtail to try and parse IP addresses as a float:

`Runtime error: strconv.ParseFloat: parsing "10.0.0.1": invalid syntax`

I have included an mtail program below that can recreate the problem along with a similar program that does not cause the problem. Curiously this seems to only occur when creating a histogram metric.

### Steps to reproduce:
1. `mtail --one_shot --progs broken.mtail --logs example.log -logtostderr`
2. Verify `Runtime error` occurs
3. `mtail --one_shot --progs working.mtail --logs example.log -logtostderr`
4. Verify program runs as expected

### `mtail --version`
```
mtail version 3.0.0-rc47 git revision 5e0099f843e4e4f2b7189c21019de18eb49181bf go version go1.16.5 go arch amd64 go os darwin
```

### AST Type Dump
I have attempted to get a type dump by running:
- `mtail --one_shot --progs broken.mtail --logs example.log --dump_ast_types --log_dir output`
- `mtail --one_shot --compile_only --progs broken.mtail --logs example.log --dump_ast_types --log_dir output`

Neither has resulted in a log beyond:

```
Log file created at: 2021/09/28 15:20:02
Running on machine: MY_MACHINE_NAME
Binary: Built with gc go1.16.5 for darwin/amd64
Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg
```

Let me know if I am running this command incorrectly and I can submit the appropriate log.

Thanks!

---

#### `example.log`
```
10.0.0.1 total_time=5
10.0.0.2 total_time=742
```

#### `broken.mtail`
```
histogram example_name buckets 1, 2 by instance

/^(?P[\d\.]+) total_time=(?P\d+)/ {
example_name[$ip] = $total / 1000.0
}
```

#### `working.mtail`
```
histogram example_name buckets 1, 2 by instance

/^(?P\d+(?:\.\d+){3}) total_time=(?P\d+)/ {
example_name[$ip] = $total / 1000.0
}
```