Add metric for pubsubshard_channels

[DriverX]

Adding gauge metric for pubsubshard_channels

pubsubshard_channels: Global number of pub/sub shard channels with client subscriptions. Added in Redis 7.0.3

[coveralls]

Pull Request Test Coverage Report for Build 198

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 91.998%

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Totals
Change from base Build 196:	0.004%
Covered Lines:	1943
Relevant Lines:	2112

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💛 - Coveralls

[oliver006]

Thanks for the PR! I'll try to review it and get it merged in the next few days.

[DriverX]

Thanks for the PR! I'll try to review it and get it merged in the next few days.

This is one line PR. Merge it please =)