add key size histogram metrics

[gojoy]

What would you like to be added?

add etcd_debugging_mvcc_key_size_bucket histogram metric.

Why is this needed?

For better observability, we can observe the distribution of key sizes and detect an increase in key size at the first time in the etcd cluster.

[gojoy]

please assign to me

[gojoy]

@jmhbnz Hi. Could you assign it to me? thinks

[jmhbnz]

/assign @gojoy

[gojoy]

I found that this problem can be solved by using irate(etcd_debugging_mvcc_total_put_size_in_bytes{}[$__interval]) / irate(etcd_mvcc_put_total{}[$__interval]). Therefore, there is no need to add new functions anymore. What do you think? @jmhbnz