Add metrics for total prefix count and ips used per cidr

[jayanthvn]

What type of PR is this?
feature

Which issue does this PR fix:
N/A

What does this PR do / Why do we need it:
Add metrics to export total prefix count and number of IPs consumer per cidr

If an issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:
N/A

Testing done on this change:

With few default pods -

# HELP awscni_total_ipv4_prefixes The total number of IPv4 prefixes\n
# TYPE awscni_total_ipv4_prefixes gauge\nawscni_total_ipv4_prefixes 1
# HELP awscni_assigned_ip_per_ipv4cidr The total number of IP addresses assigned per cidr
# TYPE awscni_assigned_ip_per_ipv4cidr gauge\nawscni_assigned_ip_per_ipv4cidr{cidr=\"192.168.42.48/28\"} 1

Added few pods in PD mode -

Deleted few pods in PD mode -

Enabled secondary IP mode with pods behind few prefixes

Deleted pods behind prefixes and prefixes got cleaned up

Scale with secondary mode -

Automation added to e2e:

N/A

Will this break upgrades or downgrades. Has updating a running cluster been tested?:
N/A

Does this change require updates to the CNI daemonset config files to work?:

N/A

Does this PR introduce any user-facing change?:

N/A

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.