Implement ContainerPilot telemetry

[misterbisson]

ContainerPilot 2.0 introduced a telemetry feature that would be very useful for monitoring this application.

The system's free RAM as a decimal from 0-1 (excluding filesystem buffers and cache):

echo "scale=2; ( $(free -m | awk '/Mem/{print $3}') - $(free -m | awk '/Mem/{print $6}') - $(free -m | awk '/Mem/{print $7}') ) / $(free -m | awk '/Mem/{print $2}')" | bc

EDIT: this is much shorter and probably more efficient:

free -m | awk 'NR==2 {printf("%.2f\n", ($3 - $6 - $7) / $2 )}'

More specific stats can be found from Memcached itself:

$ echo stats | nc 127.0.0.1 11211 
STAT pid 1222
STAT uptime 11066484
STAT time 1460951850
STAT version 1.4.4
STAT pointer_size 64
STAT rusage_user 18140.265260
STAT rusage_system 36461.835958
STAT curr_connections 24
STAT total_connections 91434
STAT connection_structures 182
STAT cmd_get 694913325
STAT cmd_set 757666708
STAT cmd_flush 0
STAT get_hits 599513796
STAT get_misses 95399529
STAT delete_misses 13041164
STAT delete_hits 429308
STAT incr_misses 0
STAT incr_hits 16047
STAT decr_misses 0
STAT decr_hits 0
STAT cas_misses 0
STAT cas_hits 0
STAT cas_badval 0
STAT auth_cmds 0
STAT auth_errors 0
STAT bytes_read 486625034028
STAT bytes_written 1339052653983
STAT limit_maxbytes 67108864
STAT accepting_conns 1
STAT listen_disabled_num 0
STAT threads 4
STAT conn_yields 0
STAT bytes 60768161
STAT curr_items 58347
STAT total_items 190874836
STAT evictions 167781776
END

From those, these items look most promising

• bytes / limit_maxbytes
• evictions (which is an internal counter, but would need to be presented as a gauge...or turned into a more useful counter)