Initial import of prometheus metrics library for orchestrator

[ameetkotian]

Related issue: #355

Description

This PR hooks up Prometheus metrics support in orchestrator.

• It uses the go-metrics-prometheus client recommended in go-metrics github page
• introduces three new config option -

	EnablePrometheusMetrics                    bool              // If true, exports prometheus style metrics in /debug/prom-metrics
	PrometheusNamespace                        string            // namespace and subsystem are components of the fully-qualified name of the metric (created by joining these components with "_")
	PrometheusSubsystem                        string            // namespace and subsystem are components of the fully-qualified name of the metric (created by joining these components with "_")

• the metrics are published at /debug/prom-metrics

Tested locally -

prom-metrics show up -

prom-metrics with PrometheusNamespace set to orc-a and PrometheusSubsystem set to bedrock -

curl -s localhost:16000/debug/prom-metrics | grep orc
# HELP orc_a_bedrock_analysis_change_write analysis.change.write
# TYPE orc_a_bedrock_analysis_change_write gauge
orc_a_bedrock_analysis_change_write 0
# HELP orc_a_bedrock_analysis_change_write_attempt analysis.change.write.attempt
# TYPE orc_a_bedrock_analysis_change_write_attempt gauge
orc_a_bedrock_analysis_change_write_attempt 0
# HELP orc_a_bedrock_audit_write audit.write
# TYPE orc_a_bedrock_audit_write gauge
orc_a_bedrock_audit_write 17
# HELP orc_a_bedrock_discoveries_attempt discoveries.attempt
# TYPE orc_a_bedrock_discoveries_attempt gauge
orc_a_bedrock_discoveries_attempt 0
# HELP orc_a_bedrock_discoveries_fail discoveries.fail
# TYPE orc_a_bedrock_discoveries_fail gauge
orc_a_bedrock_discoveries_fail 0
# HELP orc_a_bedrock_discoveries_instance_poll_seconds_exceeded discoveries.instance_poll_seconds_exceeded
# TYPE orc_a_bedrock_discoveries_instance_poll_seconds_exceeded gauge
orc_a_bedrock_discoveries_instance_poll_seconds_exceeded 0
# HELP orc_a_bedrock_discoveries_queue_length discoveries.queue_length
# TYPE orc_a_bedrock_discoveries_queue_length gauge
orc_a_bedrock_discoveries_queue_length 0
# HELP orc_a_bedrock_discoveries_recent_count discoveries.recent_count
# TYPE orc_a_bedrock_discoveries_recent_count gauge
orc_a_bedrock_discoveries_recent_count 1
# HELP orc_a_bedrock_elect_is_elected elect.is_elected
# TYPE orc_a_bedrock_elect_is_elected gauge
orc_a_bedrock_elect_is_elected 0
# HELP orc_a_bedrock_health_is_healthy health.is_healthy
# TYPE orc_a_bedrock_health_is_healthy gauge
orc_a_bedrock_health_is_healthy 1
# HELP orc_a_bedrock_instance_access_denied instance.access_denied
# TYPE orc_a_bedrock_instance_access_denied gauge
orc_a_bedrock_instance_access_denied 0