Adds 2 additional metrics

[tnosaj]

@grobie

This is a pretty small change that adds 2 new metrics for debugging errors on store operations based on the output of the stats page:

• store_no_memory

# HELP memcached_item_no_memory The number of times an item could not be stored due to no more memory.
# TYPE memcached_item_no_memory counter
memcached_item_no_memory 0

• store_too_large

# HELP memcached_item_too_large The number of times an item exceeded the max-item-size when being stored.
# TYPE memcached_item_too_large counter
memcached_item_too_large 1

I followed the format of the other metrics but was not sure about the ordering in the lists - i added these two metrics where i thought they fit most, but am happy to make any changes if needed.

Thank you!

[matthiasr]

Thank you!