[Feat] More flexible IndexerServiceMetrics

[hopeyen]

Problem statement
There's only 3 fields in the metrics tracked. It would be good to have flexibility in this struct so we can add more metrics for individual data services instead of hosting a separate metrics service.

pub struct IndexerServiceMetrics {
    pub requests: IntCounterVec,
    pub successful_requests: IntCounterVec,
    pub failed_requests: IntCounterVec,
}

Expectation proposal

Generic type/field added to IndexerServiceMetrics and function to register more metrics.

Alternative considerations
We can have a separate metrics server on the data service level, but that structure seems a bit convoluted for the data providers.