add custom metric types

[suyash]

Potential types to consider

• Counter
• Timer (Elapsed)
• Gauge

Derive
Rate (Meter)

Rate and Derive are supported implicitly by PCP, just set the reporting rate at the time unit (seconds, minutes, hours...) and set semantics to InstantSemantics to get Rate and CounterSemantics to get Derived. Gauge is a shorthand to get a float64 metric with InstantSemantics, while Counter implements a uint64 metric with CounterSemantics.

Histogram
Summary

to be considered after #28

Reservoir

couldn't find a real world example of applying reservoir sampling on metric values, why add a value if the probability of it having any real impact just keeps diminishing

see:

• https://godoc.org/github.com/heroku/instruments
• http://metrics.dropwizard.io/3.1.0/manual/core/
• https://prometheus.io/docs/concepts/metric_types/

[natoscott]

@suyash looks OK, just one issue. In PCP counter semantics, and in system performance analysis in general, counter (MMV_SEM_COUNTER) values never go backwards. I'd recommend dropping all decrement operations, because the PCP client tools will interpret this as an error (counter wrap) and report no value for that sample, which will be very confusing to the user.

Any value that can go backwards should be modeled as a gauge - in PCP, these map to instantaneous metrics (MMV_SEM_INSTANT), and then the client tools know what to do with 'em.

[coveralls]

Coverage decreased (-1.9%) to 83.124% when pulling cd16c8a on custom-metric-types into b6f56db on master.

[coveralls]

Coverage decreased (-0.2%) to 86.767% when pulling 6190476 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-0.04%) to 86.957% when pulling de1d63c on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-0.5%) to 86.477% when pulling dda4a04 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-0.5%) to 86.477% when pulling b1d3a2e on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-1.4%) to 85.576% when pulling f2cdd01 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-1.6%) to 85.439% when pulling a340c00 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-1.6%) to 85.439% when pulling b865013 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-1.6%) to 85.439% when pulling 07df091 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-2.7%) to 84.307% when pulling 24abff6 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-2.8%) to 84.237% when pulling 7fef6d9 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-3.2%) to 83.775% when pulling a33f652 on custom-metric-types into b8cc339 on master.

[coveralls]

Coverage decreased (-2.7%) to 84.337% when pulling ec80fee on custom-metric-types into b8cc339 on master.