Add metrics for SCIOND client API

[scrye]

Added metrics:

# HELP lib_sciond_conn_connections_total The amount of SCIOND connection
attempts.
# TYPE lib_sciond_conn_connections_total counter
lib_sciond_conn_connections_total{result="ok_success"} 5
# HELP lib_sciond_path_requests_total The amount of Path requests sent.
# TYPE lib_sciond_path_requests_total counter
lib_sciond_path_request_total{result="ok_success"} 4

Metrics for the other client API calls are also included.

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This change is 

[lukedirtwalker]

Reviewed 3 of 4 files at r1.
Reviewable status: 3 of 4 files reviewed, 1 unresolved discussion (waiting on @lukedirtwalker and @scrye)

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go/lib/sciond/sciond.go, line 278 at r1 (raw file):

	roundTripper, err := c.ctxAwareConnect(ctx)
	if err != nil {
		metrics.SVCInfos.Inc(errorToPrometheusLabel(err))

this is probably at the wrong place. As discussed probably it's better to do it here and before the return with nil error.

[lukedirtwalker]

Reviewed 1 of 4 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @scrye)

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go/lib/sciond/internal/metrics/metrics.go, line 79 at r1 (raw file):

func newPathRequest() Request {
	return Request{
		count: prom.NewCounterVec(Namespace, subsystemPath, "request_total",

should be requests_total

[scrye]

Reviewable status: 2 of 4 files reviewed, 2 unresolved discussions (waiting on @lukedirtwalker)

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go/lib/sciond/sciond.go, line 278 at r1 (raw file):

Previously, lukedirtwalker (Lukas Vogel) wrote…

this is probably at the wrong place. As discussed probably it's better to do it here and before the return with nil error.

Done.

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go/lib/sciond/internal/metrics/metrics.go, line 79 at r1 (raw file):

Previously, lukedirtwalker (Lukas Vogel) wrote…

should be requests_total

Done.

[lukedirtwalker]

Reviewed 2 of 2 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @scrye)

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go/lib/sciond/internal/metrics/metrics.go, line 79 at r1 (raw file):

Previously, scrye (Sergiu Costea) wrote…

Done.

Only on one metric? Doesn't it need to be everywhere?

[scrye]

Reviewable status: 3 of 4 files reviewed, 1 unresolved discussion (waiting on @lukedirtwalker)

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go/lib/sciond/internal/metrics/metrics.go, line 79 at r1 (raw file):

Previously, lukedirtwalker (Lukas Vogel) wrote…

Only on one metric? Doesn't it need to be everywhere?

Whoops, missed this one. Fixed.

[lukedirtwalker]

Reviewed 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved