Review Request 41254: Enable master get ALLOCATION_SLACK metrics.

[Guangya Liu]

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https://reviews.apache.org/r/41254/
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Review request for mesos, Artem Harutyunyan, Joris Van Remoortere, Joseph Wu, 
and Klaus Ma.


Bugs: MESOS-4124
    https://issues.apache.org/jira/browse/MESOS-4124


Repository: mesos


Description
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Enable master get ALLOCATION_SLACK metrics.


Diffs
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  src/master/master.hpp 9aa548aa6e159046c94e4ec96f631ea8b3bfd5d8 
  src/master/master.cpp 0d1482279c68f2a4a27dabaf28774769a5d515c4 
  src/master/metrics.hpp 551e4eb0f910aa4ce28a2a955019108fcd3d6371 
  src/master/metrics.cpp 5e4f4d352579dcd391a133600f9d4abc6b2402a7 

Diff: https://reviews.apache.org/r/41254/diff/


Testing
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$ curl "http://192.168.0.107:5050/metrics/snapshot"; 2>/dev/null| jq . | grep 
alloc_slack
  "master/cpus_alloc_slack_revocable_total": 3,
  "master/cpus_alloc_slack_revocable_used": 0,
  "master/disk_alloc_slack_revocable_total": 0,
  "master/disk_alloc_slack_revocable_used": 0,
  "master/mem_alloc_slack_revocable_total": 1000,
  "master/mem_alloc_slack_revocable_used": 0,


Thanks,

Guangya Liu