Prometheus metrics implementation

[eytan-avisror]

Fixes #10

This PR adds the following metrics to /metrics endpoint:

# HELP process_active_goroutines indicates the current number of active goroutines.
# TYPE process_active_goroutines gauge
process_active_goroutines 6
# HELP process_average_duration_seconds indicates the average duration of processing a hook in seconds.
# TYPE process_average_duration_seconds gauge
process_average_duration_seconds 0
# HELP process_deregistering_instances_count indicates the current number of deregistering instances.
# TYPE process_deregistering_instances_count gauge
process_deregistering_instances_count 0
# HELP process_draining_instances_count indicates the current number of draining instances.
# TYPE process_draining_instances_count gauge
process_draining_instances_count 0
# HELP process_failed_events_total indicates the sum of all failed events.
# TYPE process_failed_events_total counter
process_failed_events_total 0
# HELP process_failed_lb_deregister_total indicates the sum of all events that failed to deregister loadbalancer.
# TYPE process_failed_lb_deregister_total counter
process_failed_lb_deregister_total 0
# HELP process_failed_node_drain_total indicates the sum of all events that failed to drain the node.
# TYPE process_failed_node_drain_total counter
process_failed_node_drain_total 0
# HELP process_rejected_events_total indicates the sum of all rejected events.
# TYPE process_rejected_events_total counter
process_rejected_events_total 0
# HELP process_successful_events_total indicates the sum of all successful events.
# TYPE process_successful_events_total counter
process_successful_events_total 0
# HELP process_successful_lb_deregister_total indicates the sum of all events that succeeded to deregister loadbalancer
# TYPE process_successful_lb_deregister_total counter
process_successful_lb_deregister_total 0
# HELP process_successful_node_drain_total indicates the sum of all events that succeeded to drain the node.
# TYPE process_successful_node_drain_total counter
process_successful_node_drain_total 0
# HELP process_terminating_instances_count indicates the current number of terminating instances.
# TYPE process_terminating_instances_count gauge
process_terminating_instances_count 0

[codecov]

Codecov Report

Merging #29 into master will increase coverage by 0.36%.
The diff coverage is 83.5%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   80.06%   80.42%   +0.36%     
==========================================
  Files           8        9       +1     
  Lines         657      746      +89     
==========================================
+ Hits          526      600      +74     
- Misses         90      100      +10     
- Partials       41       46       +5

Impacted Files	Coverage Δ
pkg/service/metrics.go	100% <100%> (ø)
pkg/service/types.go	60% <38.46%> (-8.75%)	⬇️
pkg/service/server.go	77.19% <81.81%> (+0.33%)	⬆️

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