op-node: modularize metrics

The op-node currently has one big metrics struct.

With struct embedding and interface embedding we can encapsulate the metrics sub-responsibilities, and categorize this.
This will make extension of the metrics cleaner, and allow for per-chain parametrization by isolating the chain-specific kinds of metrics.

Start by locating the op-node's large metrics struct and mapping its current responsibilities, especially the chain-specific metrics. Review nearby interfaces and embedding patterns before deciding the boundaries; done means metrics responsibilities are encapsulated and chain-specific parametrization is isolated without changing intended metrics behavior.

Written by the indexing model from the issue text.