feat: Add histogram metric type

[yinggeh]

What does the PR do?

Support histogram metric type and its functions.

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• feat

Related PRs:

triton-inference-server/vllm_backend#56
triton-inference-server/core#386
triton-inference-server/server#7525

Where should the reviewer start?

n/a

Test plan:

n/a

• CI Pipeline ID:
  17487728

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Background

Customer requested histogram metrics from vLLM.

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n/a

[oandreeva-nv]

I think we need general tests for this on python backend as well

[krishung5]

+1 on the test. Also I think would be great if we can add the documentation for the new type of metrics: https://github.com/triton-inference-server/python_backend/tree/main/examples/custom_metrics

[yinggeh]

+1 on the test. Also I think would be great if we can add the documentation for the new type of metrics: https://github.com/triton-inference-server/python_backend/tree/main/examples/custom_metrics

Updated. See commit