metrics: upgraded prometheus http client

[cgbaker]

upgrade to prometheus@0.9.4 to address label conflict in Nomad 0.9.x

resolves #5345

[rgl]

Can this please be re-visisted/re-checked?

I'm also posting this to #6242:

I'm using nomad 1.0.0-beta3 and the metric label is still job instead of job_name. For example:

nomad_client_allocs_cpu_allocated{alloc_id="b15d7cc8-22d5-3e31-4a6d-096f8075abce",job="go-info",namespace="default",task="go-info",task_group="go-info"}

This means that when read from prometheus it will end up being named exported_job:

nomad_client_allocs_cpu_allocated{alloc_id="b15d7cc8-22d5-3e31-4a6d-096f8075abce",exported_job="go-info",instance="client1:4646",job="nomad",namespace="default",task="go-info",task_group="go-info"}

[github-actions]

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