Implement metrics routes

[Tchoupinax]

Hey,

Following the previous aborted try to implement the /metrics/ route, this pull request brings this feature.

Added metrics

• Default Node.js metrics
• Histogram about response duration

response_duration_bucket{le="0.005",method="GET",path="/api/v2/metrics",status="200"} 41
response_duration_bucket{le="0.01",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="0.025",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="0.05",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="0.1",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="0.25",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="0.5",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="1",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="2.5",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="5",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="10",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_bucket{le="+Inf",method="GET",path="/api/v2/metrics",status="200"} 42
response_duration_sum{method="GET",path="/api/v2/metrics",status="200"} 0.136589004
response_duration_count{method="GET",path="/api/v2/metrics",status="200"} 42

• Count of created links

# HELP link_counter Number of link created
# TYPE link_counter counter
link_counter 4

[Tchoupinax]

Hey,
It is ready to be reviewed and to bring a first valued step!
Thank you a lot

[nlamirault]

@poeti8 what do you think about this feature ?