This patch add support for the summary type on the metrics layer.

A summary is a different kind of histogram, it's buckets are percentile
so the reporting layer (i.e. Prometheus for example) would know to
report it correctly.

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

This patch adds a missing part to how histograms are being aggregated,
it needs to aggregate the sum and count as well.

Fixes scylladb#1114

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

Skip when empty, means that metrics that are not in used, will not be
reported.

A common scenario is that a user register a metric, but that metric is
never used.
The most common case is histogram and summary but it is also valid
for counters.

This patch adds an option to mark a metric with skip_when_empty.
When done so, if a metric was never used (true for histogram, counters
and summaries) it will not be reported.

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

Aggregate by labels is a mechanism for reporting aggregated results. Most
commonly it is used to report one histogram per node instead of per
shard, but it can be used for counters, and gauges.

This patch adds an option to mark a metric with a vector of labels.
That vector will be part of the metric meta-data so the reporting
layer would be able to aggregate over it.

This patch adds multiple functionality to Prometheus reporting:
1. Add summary reporting. Summaries are used in seastar to report aggregated
   percentile information (for example p95 and p99)
   The main usage is to report per-shard summary of a latency
   histograms.
2. Support aggregated metrics. With an aggregated metrics,
   Prometheus would aggregate multiple metrics based on labels and
   would report the result. Usually this would be for reporting a single
   latency histogram per node instead of per shard. But it could be used
   for counters and gauge as well.
3. Skip empty counters, histograms and summaries. It's a common practice to
   register lots of metrics even if they are not being used.
   Histograms have a huge effect on performance, so not reporting an empty
   histogram is a great performance boost both for the application and
   for the Prometheus server.
   This is true for Summaries and Counters as well, marking a metric
   with skip_when_empty would mean Prometheus will not report the
   metric if it was never used.
4. As an optimization, the stringstream that is used per metric is
   reused and clear insted of recreated.

Signed-off-by: Amnon Heiman <amnon@scylladb.com>