fix(server_family): GetMetrics should show commands in lowercase

[lsvmello]

Closes #862

I've ran INFO ALL locally and the only section that showed IN CAPS was cmdstat_COMMAND.

So I 'lowercased' all commands on GetMetrics() and fixed the replication_test.py accordingly.
Also checked that Redis behaves the same.

[kostasrim]

Hi @lsvmello! Thank you for your contribution 🚀

I left a very minor comment.

@dranikpg let me know if you had something else in mind with #862

[dranikpg]

I don't know if we want all command stats to be in lowercase, I mean that we use(d) inconsistent naming for sections in INFO ALL. Some were camel (Transaction), come were caps (STATS, MEMORY), etc

[kostasrim]

I don't know if we want all command stats to be in lowercase, I mean that we use(d) inconsistent naming for sections in INFO ALL. Some were camel (Transaction), come were caps (STATS, MEMORY), etc

Oh I see and I also did notice that this was an old issue. Anyway, their changes are only for two command stats:

# Commandstats         
cmdstat_COMMAND:calls=2,usec=50894,usec_per_call=25447             
cmdstat_INFO:calls=1,usec=2043,usec_per_call=2043

I can live with lowercase COMMAND and INFO so I personally don't mind moving forward with this PR unless there is some strong objection.

Also looking on the camelCase or CAPS it seems now that the naming of each category is coherent (First letter capital followed by all lower case) so I guess we can close the issue as well?

[lsvmello]

I've added the change requested and rebased the branch.