Add lightweight transaction metrics (aka CAS) to default dashboard

[locker]

Here's a list of metrics added for monitoring lightweight transactions performance:

1. Storage proxy layer.

   scylla_storage_proxy_coordinator_cas_read_latency
   scylla_storage_proxy_coordinator_cas_write_latency
   scylla_storage_proxy_coordinator_cas_read_timeouts
   scylla_storage_proxy_coordinator_cas_write_timeouts
   scylla_storage_proxy_coordinator_cas_read_unavailable
   scylla_storage_proxy_coordinator_cas_write_unavailable
   scylla_storage_proxy_coordinator_cas_read_unfinished_commit
   scylla_storage_proxy_coordinator_cas_write_unfinished_commit
   scylla_storage_proxy_coordinator_cas_write_condition_not_met
   scylla_storage_proxy_coordinator_cas_read_contention
   scylla_storage_proxy_coordinator_cas_write_contention
   

   See commit scylladb/scylladb@c27ab87 for more details.

2. CQL layer. New tag "conditional" has been added to a few CQL metrics. If set to "yes" for a particular metric, then the metric accounts statements that have IF conditions, otherwise - those that don't. Here's a list of metrics that now have the tag:

   scylla_cql_inserts
   scylla_cql_updates
   scylla_cql_deletes
   scylla_cql_batches
   scylla_cql_statements_in_batches
   

   See commits scylladb/scylladb@f0075ba and scylladb/scylladb@9ea8114 for more details.

[amnonh]

@tzach what is your take about this metrics?
I'm not sure about the storage proxy layer, it's good for debugging but for users it's confusing.
Maybe we should add a dashboard for cas in general,

for the cql statements, I can add a filter, that would allow selecting: everything/cas_only/non_cas

[tzach]

CAS can be added to CQL page, these are CQL operations
The overview dashboard can include one summarise metric.

I'm not sure about the storage proxy layer, it's good for debugging but for users it's confusing.

Why? we already present coordinator metrics

[amnonh]

But for the users, read/write are not clear, coordinator metrics are better taken from the CQL level.