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Support for JSONPath expressions that return arrays of values (#85)

* This is the initial implementation of support for JSONPath expressions
that return arrays of values instead of a single value.

This extends the
collector to define a few handy reducer functions that take in the slice
of float64 and return a single value. It also allows the user to define
which reducer function to use via the
"metric-config.<metricType>.<metricName>.json-path/reducer-func"
annotation, which
can have the values of 'avg', 'min', 'max' and 'sum'.

For instance, the Ruby puma webserver exposes metrics of the form of $.worker_status[*].last_status.pool_capacity that have to be consumed as an array of values to be properly targetted.

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* Renames "reducerFunc" to "aggregator" for consistency with other
collectors. Renames the annotation from
"metric-config.<metricType>.<metricName>.json-path/reducer-func" to "metric-config.<metricType>.<metricName>.json-path/aggregator".

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* Return error instead of defaulting to the avg aggregator, when no valid
aggregator name was specified and the JSONPath value is a slice of
numbers.

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* Fix index out of range on initialized output slice that was found while
writing tests.

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* Add tests for all added functions + NewJSONPathMetricsGetter

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* Add documentation on the `aggregator` option.

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* reducer function -> aggregator function

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

* Fix comment to account for returned error.

Signed-off-by: Tomás Pinho <me@tomaspinho.com>

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