feat: add aof metrics

[tomplus]

I'd like to extend list of AOF releated metrics. A server return few metrics if aof is disabled:

aof_enabled:0
aof_rewrite_in_progress:0
aof_rewrite_scheduled:0
aof_last_rewrite_time_sec:-1
aof_current_rewrite_time_sec:-1
aof_last_bgrewrite_status:ok
aof_last_write_status:ok
aof_last_cow_size:0

but if AOF is enabled we get additional interesting metrics:

127.0.0.1:6379> CONFIG set appendonly yes
OK

--
aof_enabled:1
aof_rewrite_in_progress:0
aof_rewrite_scheduled:0
aof_last_rewrite_time_sec:0
aof_current_rewrite_time_sec:-1
aof_last_bgrewrite_status:ok
aof_last_write_status:ok
aof_last_cow_size:200704
aof_current_size:92
aof_base_size:92
aof_pending_rewrite:0
aof_buffer_length:0
aof_rewrite_buffer_length:0
aof_pending_bio_fsync:0
aof_delayed_fsync:0

In this PR I added the metrics from the list above.

Thanks.

[codecov]

Codecov Report

Merging #199 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   85.02%   85.02%           
=======================================
  Files           2        2           
  Lines         621      621           
=======================================
  Hits          528      528           
  Misses         67       67           
  Partials       26       26

Impacted Files	Coverage Δ
exporter/redis.go	87.9% <ø> (ø)	⬆️

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[coveralls]

Coverage remained the same at 88.22% when pulling f99d3e9 on tomplus:feat/aof-metrics into d870a53 on oliver006:master.

[oliver006]

Thanks for the PR - looks good!

[oliver006]

Released as https://github.com/oliver006/redis_exporter/releases/tag/v0.22.1

[tomplus]

Thank you for the quick release.