Show load balancer state in master web interface

[spolitov]

Periodically we face issue when tablet leaders are not well balanced between tservers.
That would be handy to show load balancer state in this case, so that could simplify issue investigation.

[bmatican]

Once we do #1336, we can also use the revamped API to highlight this somewhere in the master UI, perhaps on the /tablet-servers page, or even in the main page! cc @rajukumaryb @rahuldesirazu

[bmatican]

I think we can probably take care of this as part of #1834.

@nspiegelberg maybe on top of just having this info in the report there, we can also just put one front and center green vs red note somewhere in the main master page?

[rahuldesirazu]

@nspiegelberg A lot of the load balancer state is tracked in cluster_balance.h

  // Circular buffer of load balancer activity.
  boost::circular_buffer<ActivityInfo> cbuf_activities_;

  // Summary of circular buffer of load balancer activity.
  int num_idle_runs_ = 0;
  std::atomic<bool> is_idle_ {true};

is_idle_ is the metric you probably want to expose as the green/red note in the master main page. For more detailed metrics, ActivityInfo is contains metadata about whether the pending task is a tserver, table, or master task.

To actually expose the metrics, you can modify master-path-handlers.h/cc.

Hope this helps!

[nspiegelberg]

[nspiegelberg]

@yb-andrew : for observing my puny X and trying to make it big and red

[nspiegelberg]

BIG RED X!!! Success still looks boring