feat: add node role metric

[coveralls]

Pull Request Test Coverage Report for Build 165

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 92.538%

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Totals
Change from base Build 164:	0.02%
Covered Lines:	1947
Relevant Lines:	2104

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💛 - Coveralls

[oliver006]

instance_info already exports role - how is this different?

See here:

redis_instance_info{app="redis", instance="10.42.0.246:9121", job="kubernetes-pods", maxmemory_policy="noeviction", 
namespace="redis", os="Linux 5.4.0-153-generic x86_64", pod="redis-5d7898859b-6jnq9", 
pod_template_hash="5d7898859b", process_id="1", redis_build_id="3a1fe9d08e5ebeb", redis_mode="standalone", 
redis_version="6.2.12", role="master", run_id="9a291d29d3ceea449633316f29712616ecc11a07", tcp_port="6379"}

[snoojen]

Thanks for the response! That fits the need we have for now - I'll close the PR.