Refactor metrics server to be multi-tenant

[mostafa]

• Investigate if it is feasible: it is feasible, yet it needs more work on plugins, which should go into the SDK and/or individual plugins.
• Implement: I've made a WIP PR in v0.7.x milestone and I explained the further work needed to make it work properly. I'll remove it from this milestone and will continue the work when this has higher demand/priority.

gatewayd/cmd/run.go

Lines 160 to 161 in e2a725e

	// TODO: Start multiple metrics servers. For now, only one default is supported.
	// I should first find a use case for those multiple metrics servers.