[Feature Request] Support get slowlog in redis

## Feature Request

Get slow log command from redis.

Example command:

```
> SLOWLOG GET
1) 1) (integer) 10595463
2) (integer) 1490323467
3) (integer) 8831
4) 1) "HGETALL"
2) "telegraf:metric:redis"
```
- A unique progressive identifier for every slow log entry.
- The unix timestamp at which the logged command was processed. (important)
- The amount of time needed for its execution, in microseconds. (important)
- The array composing the arguments of the command. (important)

Ref https://redis.io/commands/slowlog

### Use case:

Monitoring slow query in redis, time duration of command and count slow command


No file or test is named in the issue. Start by reading Redis's SLOWLOG GET reference and locating Telegraf's existing Redis input entry point; done should expose the slow-log identifier, timestamp, duration, and command arguments as monitoring data.

Written by the indexing model from the issue text.