FORWARDER_PROM_NAMESPACE must be validated

[mmatczuk]

Using custom prom namespace can lead to panic

forwarder-e2e-upstream-proxy-1  | panic: descriptor Desc{fqName: "upstream-proxy_http_requests_total", help: "Total number of HTTP requests processed.", constLabels: {}, variableLabels: [{code <nil>} {method <nil>} {host <nil>} {source <nil>}]} is invalid: "upstream-proxy_http_requests_total" is not a valid metric name
forwarder-e2e-upstream-proxy-1  | 
forwarder-e2e-upstream-proxy-1  | 
forwarder-e2e-upstream-proxy-1  | goroutine 1 [running]:
forwarder-e2e-upstream-proxy-1  | github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(0x40002b0620?, {0x40002b5450?, 0x1, 0x0?})
forwarder-e2e-upstream-proxy-1  |       /Users/michal/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/registry.go:405 +0x84
forwarder-e2e-upstream-proxy-1  | github.com/prometheus/client_golang/prometheus/promauto.Factory.NewCounterVec({{0xa41320?, 0x40002b80f0?}}, {{0x40002b0620, 0xe}, {0x0, 0x0}, {0x90f564, 0x13}, {0x922bf0, 0x28}, ...}, ...)
forwarder-e2e-upstream-proxy-1  |       /Users/michal/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/promauto/auto.go:276 +0x178
forwarder-e2e-upstream-proxy-1  | github.com/saucelabs/forwarder/middleware.NewPrometheus({0xa41320?, 0x40002b80f0?}, {0x40002b0620, 0xe})
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/middleware/prometheus.go:58 +0x1cc
forwarder-e2e-upstream-proxy-1  | github.com/saucelabs/forwarder.(*HTTPProxy).middlewareStack(0x40002ce380)
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/http_proxy.go:361 +0x4ec
forwarder-e2e-upstream-proxy-1  | github.com/saucelabs/forwarder.(*HTTPProxy).configureProxy(0x40002ce380)
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/http_proxy.go:287 +0xb5c
forwarder-e2e-upstream-proxy-1  | github.com/saucelabs/forwarder.newHTTPProxy(0x40002de000, {0x0, 0x0}, 0x0, {0xa3a3e0, 0x40002bc500}, {0xa412f0?, 0x40002ab650})
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/http_proxy.go:203 +0x2f8
forwarder-e2e-upstream-proxy-1  | github.com/saucelabs/forwarder.NewHTTPProxy(0x8b4860?, {0x0?, 0x0?}, 0x906276?, {0xa3a3e0?, 0x40002bc500?}, {0xa412f0?, 0x40002ab650?})
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/http_proxy.go:146 +0x34
forwarder-e2e-upstream-proxy-1  | github.com/saucelabs/forwarder/cmd/forwarder/run.(*command).runE(0x40002a8300, 0x40002da300, {0x0?, 0x0?, 0x0?})
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/cmd/forwarder/run/run.go:169 +0xcf8
forwarder-e2e-upstream-proxy-1  | github.com/spf13/cobra.(*Command).execute(0x40002da300, {0x117c080, 0x0, 0x0})
forwarder-e2e-upstream-proxy-1  |       /Users/michal/go/pkg/mod/github.com/spf13/[email protected]/command.go:940 +0x658
forwarder-e2e-upstream-proxy-1  | github.com/spf13/cobra.(*Command).ExecuteC(0x40002da000)
forwarder-e2e-upstream-proxy-1  |       /Users/michal/go/pkg/mod/github.com/spf13/[email protected]/command.go:1068 +0x320
forwarder-e2e-upstream-proxy-1  | github.com/spf13/cobra.(*Command).Execute(...)
forwarder-e2e-upstream-proxy-1  |       /Users/michal/go/pkg/mod/github.com/spf13/[email protected]/command.go:992
forwarder-e2e-upstream-proxy-1  | main.main()
forwarder-e2e-upstream-proxy-1  |       /Users/michal/Sauce-CONNECT-5/forwarder/cmd/forwarder/main.go:14 +0x20