Log total memory in Sidekiq Memory killer

What does this MR do and why?

Adds MemoryTotal to logs when sidekiq memory killer kick in. We are currently using hard-coded soft limit, specified with ENV variable SIDEKIQ_MEMORY_KILLER_MAX_RSS, which is configured for each sidekiq shard separately. We could use memory total information to make sidekiq Memory Killer smarter, by calculating the limit automatically (i.e 70% of total memory)

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Related to: #374755 (closed)