feature/BCF-2604-metrics: instrumenting prometheus metrics

[patrickhuie19]

Instruments default go prometheus metrics.

Note: And bumps go to 1.20.0

go 1.17.0 is required for the /x/sys library. I bumped to 1.20.0 to match the go version of /smartcontractkit/chainlink

Library consumers can obtain metrics by running the new ServeMetrics API exposed by Server. Like Serve, ServeMetrics is blocking and the resources associated with the metrics server are closed by the done signal.

The default endpoint for the metrics is :2112/metrics, though the port can be configured by the consumer.

To test, checkout examples/simple/server and run go run . in one terminal. In a separate terminal run curl http://localhost:2112/metrics. The beginning of the output should look like the following:

wsrpc % curl http://localhost:2112/metrics
# HELP go_gc_duration_seconds A summary of the pause duration of garbage collection cycles.
# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{quantile="0"} 0
go_gc_duration_seconds{quantile="0.25"} 0
go_gc_duration_seconds{quantile="0.5"} 0
go_gc_duration_seconds{quantile="0.75"} 0
go_gc_duration_seconds{quantile="1"} 0
go_gc_duration_seconds_sum 0
go_gc_duration_seconds_count 0
# HELP go_goroutines Number of goroutines that currently exist.
# TYPE go_goroutines gauge
go_goroutines 14
...
...