feat: Bump prometheus-client upperbound to 0.19.0

[paiyar]

Background

Proposing that we bump the upper bound of the prometheus_client library required by the seldon-core python sdk to include the latest 0.19.0

Reasoning

We have projects that use the latest promethues_client library and are unable to use this library due to version restrictions:

pip._vendor.resolvelib.resolvers.ResolutionImpossible: [RequirementInformation(requirement=SpecifierRequirement('prometheus-client==0.17.1'), parent=None), RequirementInformation(requirement=SpecifierRequirement('prometheus-client<0.9.0,>=0.7.1'), parent=LinkCandidate('https://files.pythonhosted.org/packages/46/0a/261128eeae231b024aa0acf4f633be4c5b1f674e089298be360f338345b8/seldon_core-1.17.1-py3-none-any.whl (from https://pypi.org/simple/seldon-core/) (requires-python:>=3.6)'))]

[seldondev]

Hi @paiyar. Thanks for your PR.

I'm waiting for a SeldonIO or todo member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[CLAassistant]

All committers have signed the CLA.

[lc525]

/ok-to-test