Metric family metadata consists of information relevant to all metrics
with the same name (but with different labels).  For example, name,
description, and aggregate labels.

The metric family metadata is found in two data structures: the
_value_map that holds all registered metrics and _metadata that contains
only reported metrics.

This patch makes the _metadata hold a reference to the data in the
_value_map instead of copying it, which serves two purposes:
1. Minor performance gain (memory and computation).
2. It will now be possible for a change in _value_map to be reflected in
   _metadata.

Signed-off-by: Amnon Heiman <[email protected]>

This patch adds support for metric family config.
A metric family config is a configuration that relates to all the
metrics with the same name (but with different labels).

The configuration would allow changing the labels aggregation for
metrics.

Besides the configuration class, which is similar to
metrics_relabel_config, two enhancements were added to the
relabel_config_regex:

1. empty() which check if it's empty or not.
2. match() that checks if the regex is nonempty and matches a string
without returning the matched group.

Signed-off-by: Amnon Heiman <[email protected]>

Family config is a configuration that relates to all metrics with the
same name but with different labels values.

set_metric_family_configs allows changing that configuration during run
time. Specifically, change the label aggregation based on a metric name.

The following is an example for setting the aggregate labels for the
metric test_gauge_1 and all metrics matching the regex test_gauge1.*:

std::vector<sm::metric_family_config> fc(2);
fc[0].name = "test_gauge_1";
fc[0].aggregate_labels = { "lb" };
fc[1].regex_name = "test_gauge1.*";
fc[1].aggregate_labels = { "ll", "aa" };

sm::set_metric_family_configs(fc);

Signed-off-by: Amnon Heiman <[email protected]>

include/seastar/core/metrics_api.hh

This patch adds a test for the family config API. it register metrics
and change their aggregate labels.
It check that both metrics that create before and after the call to
set_metric_family_configs works as expected.

Signed-off-by: Amnon Heiman <[email protected]>