Switch to a fail fast approach when creating duplicate metrics

[fab-10]

PR description

Switch to a fail fast approach when trying to create a metric that already exists, that is less error prone of the current approach where instead in case the same metric is created twice, the first one is replaced to the second one, thus in case of a name clash that could have unwanted consequences, so better to fail in case this happens.

This change required a little refactoring to make sure metrics collectors are created only one in the RPC subsystem, with the introduction of the RpcMetrics and for Vert.x with the introduction of the VertxMetricsCollectors.

Fixed Issue(s)

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