Review validator monitor usage

we track the publish block delay in validator monitor currently, could do the same for envelopes, but validator monitor seems the wrong place to track timing info like this, we can keep the stub for now and revisit later, just realized there is still a `registerBlobSidecar` which is similar and doesn't really belong in the validator monitor

_Originally posted by @nflaig in https://github.com/ChainSafe/lodestar/pull/8962#discussion_r2911464689_