Make prometheus metrics configurable

[thinkharderdev]

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In #511 we've added prometheus metrics in the default build of the ballista scheduler but the metrics exposed (and more importantly the histogram buckets used) are currently hard-coded.

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Allow users to configure metrics. This should be along two dimensions:

1. We may support an increasing set of metrics which we can track, but users may only want to track a subset of them. In that case they should be able to toggle on/off metrics.
2. For histogram metrics, they should be able to configure the histogram buckets.

Describe alternatives you've considered
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Users can always implement a custom SchedulerMetricsCollector and do whatever they want, but it is pain since they will not be able to use the standard image and will have to build their project themselves and manage their own entrypoint.

Alternatively, we could somehow leverage a plugin mechanism for this

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