Optional metric for era points of specified validator

[ErakhtinB]

Referenced issues

If validator-id is provided by cli, metric of it's era points in current era appears. Info about active era, list of validators in it and their points are taken from storage. Once a minute it's set in gauge in same registry as other metrics. If id is not validator anymore it's era points number is considered 0.

Description of the Change

Possible Drawbacks

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[igor-egorov]

I strongly disagree with the case of sptr being initialized with nullptr.
A possible solution could be wrapping it with an optional.

[kamilsa]

I agree. Signature of the function could be just optional<shared_ptr<...>>

[kamilsa]

I agree. Signature of the function could be just optional<shared_ptr<...>>