[metrics] add effective proposer slot to the metrics

[tsachiherman]

Context and scope
The goal of this issue is to add a metrics gauge(s) that would enable the tracking of the effective proposer slot.
In order to achieve that, we'll be adding 2 metrics:

1. A metric showing when my local node is going to attempt to propose the block (either by showing the slot index for the next block or by showing how much time I'm waiting until I'm the block producer or something)
2. A metric counting the slot indices of accepted blocks

Discussion and alternatives

1. In order to accomplish the first goal, we'll add a gauge during SetPrefrence. That might be a bit noisy, as it would report both selected and non-selected slots.
2. Create a slots histogram during the block Accept, allowing us to collect statistics.

Open questions