add missing redis metrics

[phemmer]

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This makes sure that all redis metrics are present without having to use a hard-coded list of what metrics to pull in.
Fixes #1685

I did opt for skipping some stuff, such as most of the "server" section, with the exception of 2 metrics. Most of the fields in this section aren't metrics, but server info. Also skipped any "*_human" field since each has a more accurate field available.

For the fields that were being renamed, I kept the renaming behavior.

I also added a rdb_last_save_time_elapsed metric which is calculated using rdb_last_save_time, which is an absolute timestamp, and the current time.
Influxdb doesn't support select time - rdb_last_save_time, so without this, there's no way to tell how long ago the last save was.

[sparrc]

thanks @phemmer