Deprecate expvar metrics backend

[yurishkuro]

Which problem is this PR solving?

• Step 1 of [deprecation] Remove support for expvar metrics #4722

Description of the changes

• add deprecation notice for expvar as metrics backend
• change HotROD default backend to prometheus

How was this change tested?

• run binaries locally to observe the messages

$ go run ./cmd/all-in-one help
      --metrics-backend string                                    Defines which metrics backend to use for metrics reporting: prometheus, none, or expvar (deprecated, will be removed after 2024-01-01 or in release v1.53.0, whichever is later)  (default "prometheus")

$ go run ./cmd/all-in-one --metrics-backend=expvar
2023/09/09 19:09:01 using expvar as metrics backend (deprecated, will be removed after 2024-01-01 or in release v1.53.0, whichever is later)

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
internal/metrics/metricsbuilder/builder.go	100.00%

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