Prometheus http and kernel startup/shutdown metrics

[MaicoTimmerman]

This MR is taking on #731.

The jupyter_server already ships with a PrometheusMetricsHandler, however, that handler inherits from JupyterHandler, which requires authentication. The enterprise gateway project doesn't integrate with that method of authentication, therefore I added separate handler to serve the metrics.

I've included 3 metrics in the initial implementation:

• All HTTP response timings per handler, status_code and method
• Kernel startup duration in seconds
• Kernel shutdown duration in seconds

In terms of configuration, I've included the EG_METRICS_PREFIX environment variable for now, similar to how other configurations for the process proxies are set.

Sample responses from the /metrics endpoint

enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.005",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.01",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.025",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.05",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.075",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.1",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.25",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.5",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="0.75",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="1.0",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="2.5",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="5.0",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="7.5",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="10.0",method="POST",status_code="200"} 0.0
enterprise_gateway_http_request_duration_seconds_bucket{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",le="+Inf",method="POST",status_code="200"} 1.0
enterprise_gateway_http_request_duration_seconds_count{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",method="POST",status_code="200"} 1.0
enterprise_gateway_http_request_duration_seconds_sum{handler="enterprise_gateway.services.kernels.handlers.KernelActionHandler",method="POST",status_code="200"} 19.83746862411499

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[MaicoTimmerman]

Read the docs build failed due to timeouts:

    RuntimeError: Download error (28) Timeout was reached [https://conda.anaconda.org/free/noarch/repodata.json]
    Operation too slow. Less than 30 bytes/sec transferred the last 60 seconds