feat: Add sampler metrics

[kentquirk]

Which problem is this PR solving?

• Adds sampler-specific metrics now that dynsampler-go supports them.

Short description of the changes

• Add getMetricType function to get the type from a metric by its name. This is required because Metrics can only increment counters, not set them to a value, so we need to treat metrics ending in _count specially by keeping track of their previous value so we can get a delta.
• Register all metrics provided by the sampler
• Record them on every call to GetSampleRate
• Add metrics to DeterministicSampler following the same pattern
• Update rules to do the same (and add support for new downstream sampler types)

Closes #704.
Closes #554.