[DONT MERGE]Document bouncer metrics

[sbs2001]

Documents: https://github.com/crowdsecurity/cs-blocklist-mirror/pull/13 https://github.com/crowdsecurity/cs-cloudflare-bouncer/pull/110 https://github.com/crowdsecurity/cs-firewall-bouncer/pull/189

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[LaurenceJJones]

Can we have some clarification if this PR is still needed as most have been updated