redis input: fix percentage value parsing

[melor]

Percentage values would get interpreted incorrectly as tags because they
did not pass the int/float conversion.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[melor]

Before: used_memory_dataset_perc and used_memory_peak_perc get included as tags for all metrics:

redis_used_cpu_sys{aof_last_bgrewrite_status="ok",aof_last_write_status="ok",cloud="google-europe-west1",host="redis1-1",maxmemory_policy="noeviction",port="19044",project="test",rdb_last_bgsave_status="ok",redis_version="5.0.4",replication_role="master",server="fda7:a938:5bfe:5fa6::1",service="redis1",service_type="redis",used_memory_dataset_perc="20.54%",used_memory_peak_perc="91.40%"} 9.739602

redis_used_cpu_sys_children{aof_last_bgrewrite_status="ok",aof_last_write_status="ok",cloud="google-europe-west1",host="redis1-1",maxmemory_policy="noeviction",port="19044",project="test",rdb_last_bgsave_status="ok",redis_version="5.0.4",replication_role="master",server="fda7:a938:5bfe:5fa6::1",service="redis1",service_type="redis",used_memory_dataset_perc="20.54%",used_memory_peak_perc="91.40%"} 0.001035

After: used_memory_dataset_perc and used_memory_peak_perc are separate outputs and no longer as tags:

redis_used_cpu_user{aof_last_bgrewrite_status="ok",aof_last_write_status="ok",cloud="google-europe-west1",host="redis1-1",maxmemory_policy="noeviction",port="19044",project="test",rdb_last_bgsave_status="ok",redis_version="5.0.4",replication_role="master",server="fda7:a938:5bfe:5fa6::1",service="redis1",service_type="redis"} 9.907533

redis_used_cpu_user_children{aof_last_bgrewrite_status="ok",aof_last_write_status="ok",cloud="google-europe-west1",host="redis1-1",maxmemory_policy="noeviction",port="19044",project="test",rdb_last_bgsave_status="ok",redis_version="5.0.4",replication_role="master",server="fda7:a938:5bfe:5fa6::1",service="redis1",service_type="redis"} 0

redis_used_memory_peak_perc{aof_last_bgrewrite_status="ok",aof_last_write_status="ok",cloud="google-europe-west1",host="redis1-1",maxmemory_policy="noeviction",port="19044",project="test",rdb_last_bgsave_status="ok",redis_version="5.0.4",replication_role="master",server="fda7:a938:5bfe:5fa6::1",service="redis1",service_type="redis"} 91.4

redis_used_memory_dataset_perc{aof_last_bgrewrite_status="ok",aof_last_write_status="ok",cloud="google-europe-west1",host="redis1-1",maxmemory_policy="noeviction",port="19044",project="test",rdb_last_bgsave_status="ok",redis_version="5.0.4",replication_role="master",server="fda7:a938:5bfe:5fa6::1",service="redis1",service_type="redis"} 20.57

Example values queries thru the Prometheus output interface.

[GeorgeMac]

Cheers for the contribution @melor !

Would you mind adding a case to the tests to validate this?

[melor]

Related: #5036