Added HealthMetrics and tests for the HealthMetrics and PendingTrace classes

[nayeem-kamal]

What Does This Do

• Created a new branch/PR because rebasing the old one caused it to show 280+ files changed.

Added the following health metrics:

Count of traces

• Created trace.pending.created

Count of spans

• Created - span.pending.created
• Sampled - tracing.context.size (already existed)

Count of continuations

• Finished - span.continuations.finished
• Canceled - span.continuations.canceled

Motivation

Additional Notes

Updated List of metrics

Count of traces

• Created - trace.pending.created
• Sampled Here - queue.enqueued.traces/queue.dropped.traces - SAMPLER_KEEP tag
• w/ Errors - queue.dropped.traces/api.errors.total
• Sent - flush.traces.total

Count of spans

• Created - span.pending.created
• Sampled - tracing.context.size

Count of continuations

• Finished - span.continuations.finished
• Canceled - span.continuations.canceled