[TASK] add Prometheus

[genofire]

to run yanic with promtheus, you need both parts

• [webserver.prometheus] <- the exporter
• [[nodes.output.prometheus-sd]] <- service discovery to write json

example prometheus config to map the exporter (like blackbox-exporter)

  - job_name: 'yanic'
    metrics_path: /metric
    file_sd_configs:
      - files:
          - '/tmp/yanic.json'
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_node_id
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: EXPORT_WEBSERVER:PORT
      - source_labels: [job]
        target_label: service

[codecov-io]

Codecov Report

Merging #177 into master will decrease coverage by 1.84%.
The diff coverage is 56.39%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
- Coverage   75.05%   73.21%   -1.85%     
==========================================
  Files          52       56       +4     
  Lines        1772     1975     +203     
==========================================
+ Hits         1330     1446     +116     
- Misses        404      491      +87     
  Partials       38       38

Impacted Files	Coverage Δ
cmd/serve.go	7.69% <0%> (-1.4%)	⬇️
webserver/prometheus/exporter.go	0% <0%> (ø)
webserver/prometheus/config.go	0% <0%> (ø)
webserver/webserver.go	100% <100%> (ø)	⬆️
webserver/prometheus/transform.go	100% <100%> (ø)
webserver/prometheus/metric.go	100% <100%> (ø)