Adding metrics endpoint for prometheus scraping

[wileyj]

This adds a metrics endpoint for the app on port 9153 using promster. Why? When working with prometheus, it's sometimes easier to have a small app you can scrape from where the data is consistent.

This PR adds a very basic prometheus server to the application, exposing metrics server on port 9153 that can be requested: curl -s localhost:9153.

It may be out of scope from the original intent of this repo, but I've found these changes useful and can imagine others might as well.

Example output:

$ curl -s localhost:80/test
{
  "path": "/test",
  "headers": {
    "host": "localhost",
    "user-agent": "curl/7.64.1",
    "accept": "*/*"
  },
  "method": "GET",
  "body": "",
  "fresh": false,
  "hostname": "localhost",
  "ip": "::ffff:127.0.0.1",
  "ips": [],
  "protocol": "http",
  "query": {},
  "subdomains": [],
  "xhr": false,
  "os": {
    "hostname": "wileyj"
  },
  "connection": {}
}

$ curl -s localhost:9153
# HELP process_cpu_user_seconds_total Total user CPU time spent in seconds.
# TYPE process_cpu_user_seconds_total counter
process_cpu_user_seconds_total 0.12645199999999998 1630078381013

# HELP process_cpu_system_seconds_total Total system CPU time spent in seconds.
# TYPE process_cpu_system_seconds_total counter
process_cpu_system_seconds_total 0.017154 1630078381013

# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 0.14360599999999998 1630078381013

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# TYPE process_start_time_seconds gauge
process_start_time_seconds 1630078330

# HELP process_resident_memory_bytes Resident memory size in bytes.
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process_resident_memory_bytes 35831808 1630078381013

# HELP nodejs_eventloop_lag_seconds Lag of event loop in seconds.
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nodejs_eventloop_lag_seconds 0.000600628 1630078381014

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nodejs_active_requests_total 0 1630078381013

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# HELP nodejs_heap_space_size_used_bytes Process heap space size used from node.js in bytes.
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nodejs_gc_runs_total{gc_type="mark_sweep_compact"} 2

# HELP nodejs_gc_pause_seconds_total Time spent in GC Pause in seconds.
# TYPE nodejs_gc_pause_seconds_total counter
nodejs_gc_pause_seconds_total{gc_type="scavenge"} 0.002521724
nodejs_gc_pause_seconds_total{gc_type="incremental_marking"} 0.0009807589999999999
nodejs_gc_pause_seconds_total{gc_type="mark_sweep_compact"} 0.014777158

# HELP nodejs_gc_reclaimed_bytes_total Total number of bytes reclaimed by GC.
# TYPE nodejs_gc_reclaimed_bytes_total counter
nodejs_gc_reclaimed_bytes_total{gc_type="scavenge"} 2334184
nodejs_gc_reclaimed_bytes_total{gc_type="mark_sweep_compact"} 2117608

# HELP http_request_duration_seconds The HTTP request latencies in seconds.
# TYPE http_request_duration_seconds histogram
http_request_duration_seconds_bucket{le="0.05",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="0.1",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="0.3",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="0.5",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="0.8",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="1",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="1.5",method="get",status_code="200",path="/"} 2
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http_request_duration_seconds_bucket{le="+Inf",method="get",status_code="200",path="/"} 2
http_request_duration_seconds_sum{method="get",status_code="200",path="/"} 0.016163167
http_request_duration_seconds_count{method="get",status_code="200",path="/"} 2
http_request_duration_seconds_bucket{le="0.05",method="get",status_code="200",path="/test"} 1
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http_request_duration_seconds_bucket{le="0.3",method="get",status_code="200",path="/test"} 1
http_request_duration_seconds_bucket{le="0.5",method="get",status_code="200",path="/test"} 1
http_request_duration_seconds_bucket{le="0.8",method="get",status_code="200",path="/test"} 1
http_request_duration_seconds_bucket{le="1",method="get",status_code="200",path="/test"} 1
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http_request_duration_seconds_bucket{le="+Inf",method="get",status_code="200",path="/test"} 1
http_request_duration_seconds_sum{method="get",status_code="200",path="/test"} 0.003469653
http_request_duration_seconds_count{method="get",status_code="200",path="/test"} 1

# HELP http_requests_total The total HTTP requests.
# TYPE http_requests_total counter
http_requests_total{method="get",status_code="200",path="/"} 2
http_requests_total{method="get",status_code="200",path="/test"} 1

[mendhak]

Apologies for late reply. Sorry and as you guessed, this is out of scope or intention of this tool, and it adds another layer of maintenance/overhead for me.

[wileyj]

no problem at all, thanks for taking a look