Deprecate the `telemetry.disableHighCardinalityMetrics` feature gate

[axw]

Component(s)

service

Describe the issue you're reporting

The telemetry.disableHighCardinalityMetrics feature gate is currently in alpha, meaning it is disabled by default, i.e. high cardinality metrics are enabled by default (double negative).

In #12433, metric views were made configurable. It is not permitted to simultaneously configure views and enable the feature gate -- the service will exit with the error "telemetry.disableHighCardinalityMetrics gate is incompatible with service::telemetry::metrics::views".

The feature gate is commented to say "The gate will be removed when the collector allows for view configuration. "

Given that the collector now allows for view configuration, we should remove the gate. Before we do that, we should deprecate the flag to give users a bit of warning.

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