cni-metrics-helper metrics: do type assertion before type casting

[dshehbaj]

What type of PR is this?

Bug fix

Which issue does this PR fix?:

#3144

Testing done on this change:

Added unit tests for code path for publishing CloudWatch and Prometheus Metrics and also pasted debug logs to confirm that no panic happens.

Built metrics helper image successfully using make docker-metrics.

Unit tests passing

// Cloud watch unit test

$> go test -timeout 30s -run ^TestProduceCloudWatchMetrics$ github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics

ok  	github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics


// Prometheus Unit Test

$> go test -timeout 30s -run ^TestProducePrometheusMetrics$ github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics

ok  	github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics

Debug logs before the change

// LIST MAINTAINED BY CNI METRICS HELPER

{"msg":"Printing out all the Prometheus Mappings"}
{"msg":"Metric Name: awscni_eni_max, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_del_ip_req_count, Metric Type: *prometheus.CounterVec"}
{"msg":"Metric Name: awscni_total_ip_addresses, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_assigned_ip_addresses, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_no_available_ip_addresses, Metric Type: *prometheus.counter"}
{"msg":"Metric Name: awscni_total_ipv4_prefixes, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_ip_max, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_add_ip_req_count, Metric Type: *prometheus.counter"}
{"msg":"Metric Name: awscni_eni_allocated, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_force_removed_enis, Metric Type: *prometheus.counter"}
{"msg":"Metric Name: awscni_force_removed_ips, Metric Type: *prometheus.counter"}

// LIST RECIEVED FROM CNI

{"msg":"Printing out all the families recieved"}
{"msg":"Metric Name: awscni_ipamd_action_inprogress, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_reconcile_count, Metric Type: COUNTER"}
{"msg":"Metric Name: awscni_assigned_ip_per_cidr, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_ip_max, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_aws_api_latency_ms, Metric Type: SUMMARY"}
{"msg":"Metric Name: awscni_force_removed_enis, Metric Type: GAUGE"}
panic: interface conversion: *prometheus.counter is not prometheus.Gauge: missing method Dec

goroutine 1 [running]:
github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics.producePrometheusMetrics({0x35e8190, 0xc0004a6960}, 0xc0004a2150, 0xc0001aea50)
        /go/src/github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics/metrics.go:393 +0xce5
github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics.Handler({0x35e2690?, 0xc0001d7090?}, {0x35e8190, 0xc0004a6960})
        /go/src/github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/metrics/metrics.go:479 +0x16a
main.main()
        /go/src/github.com/aws/amazon-vpc-cni-k8s/cmd/cni-metrics-helper/main.go:176 +0xf7c

Debug logs after the change

// LIST MAINTAINED BY CNI METRICS HELPER

{"msg":"Printing out all the Prometheus Mappings"}
{"msg":"Metric Name: awscni_eni_max, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_total_ip_addresses, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_force_removed_ips, Metric Type: *prometheus.counter"}
{"msg":"Metric Name: awscni_total_ipv4_prefixes, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_ip_max, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_add_ip_req_count, Metric Type: *prometheus.counter"}
{"msg":"Metric Name: awscni_del_ip_req_count, Metric Type: *prometheus.CounterVec"}
{"msg":"Metric Name: awscni_eni_allocated, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_assigned_ip_addresses, Metric Type: *prometheus.gauge"}
{"msg":"Metric Name: awscni_force_removed_enis, Metric Type: *prometheus.counter"}
{"msg":"Metric Name: awscni_no_available_ip_addresses, Metric Type: *prometheus.counter"}
{"msg":"\n\n"}

// LIST RECIEVED FROM CNI

{"msg":"Printing out all the families recieved"}
{"msg":"Metric Name: awscni_eni_max, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_assigned_ip_per_cidr, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_ipamd_action_inprogress, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_assigned_ip_addresses, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_reconcile_count, Metric Type: COUNTER"}
{"msg":"Metric Name: awscni_eni_allocated, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_del_ip_req_count, Metric Type: COUNTER"}
{"msg":"Metric Name: awscni_ec2api_req_count, Metric Type: COUNTER"}
{"msg":"Metric Name: awscni_ec2api_error_count, Metric Type: COUNTER"}

// TYPE ASSERTION CHECK
{"msg":"Metric Name: awscni_force_removed_ips, Metric Type: GAUGE"}
{"msg":"Metric awscni_force_removed_ips is not a Gauge type, skipping"}

// TYPE ASSERTION CHECK
{"msg":"Metric Name: awscni_force_removed_enis, Metric Type: GAUGE"}
{"msg":"Metric awscni_force_removed_enis is not a Gauge type, skipping"}

{"msg":"Metric Name: awscni_ip_max, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_aws_api_latency_ms, Metric Type: SUMMARY"}
{"msg":"Metric Name: awscni_aws_api_error_count, Metric Type: GAUGE"}

// TYPE ASSERTION CHECK
{"msg":"Metric Name: awscni_add_ip_req_count, Metric Type: GAUGE"}
{"msg":"Metric awscni_add_ip_req_count is not a Gauge type, skipping"}

{"msg":"Metric Name: awscni_total_ipv4_prefixes, Metric Type: GAUGE"}
{"msg":"Metric Name: awscni_total_ip_addresses, Metric Type: GAUGE"}


// NO PANIC HAPPENS