Monitoring spanmetrics (#797)

* wip cache metric from spanmetrics

* testing

* testing

* WIP: collecting metrics

* metric name updated

* Update exporter/prometheusexporter/prometheus.go

Co-authored-by: tanner-bruce <b.r.uce.tanner@gmail.com>

* Update processor/spanmetricsprocessor/processor.go

Co-authored-by: tanner-bruce <b.r.uce.tanner@gmail.com>

* wip error metrics

* span metrics monitoring : refactored internal metrics counters

* span metrics monitoring : added metricKey error

* Update processor/spanmetricsprocessor/processor.go

Co-authored-by: tanner-bruce <b.r.uce.tanner@gmail.com>

* Update processor/spanmetricsprocessor/processor.go

Co-authored-by: tanner-bruce <b.r.uce.tanner@gmail.com>

* moved metric one loop up

* unique metrics count

* removed actice timeseries counter

* Update processor/spanmetricsprocessor/processor.go

Co-authored-by: tanner-bruce <b.r.uce.tanner@gmail.com>

Co-authored-by: tanner-bruce <b.r.uce.tanner@gmail.com>

Author: arun-shopify

go.mod

@@ -368,7 +368,7 @@ require (
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/power-devops/perfstat v0.0.0-20210106213030-5aafc221ea8c // indirect
 	github.com/pquerna/cachecontrol v0.1.0 // indirect
-	github.com/prometheus/client_golang v1.12.1 // indirect
+	github.com/prometheus/client_golang v1.12.2-0.20220318110013-3bc8f2c651ff // indirect
 	github.com/prometheus/client_model v0.2.0 // indirect
 	github.com/prometheus/common v0.32.1 // indirect
 	github.com/prometheus/common/sigv4 v0.1.0 // indirect