Support item count and operation name in azmonexporter

[trask]

Introduces

• AzureMonitorSpanProcessor - handles operation name propagation
• AzureMonitorSampler - implements Azure Monitor trace-id hashing based sampler

Removes

• Agent's AiSampler which used tracestate to propagate sampling decision (and so required coordination with propagator to suppress propagating that tracestate since it wasn't a standard so didn't want to expose that publicly)

Fixes

• Agent's default sampling behavior to not be "parent-based" because that doesn't support accurate "item counts" since item count isn't propagated in tracestate (hopefully some day)
• Replaces configuration option ignoreRemoteParentNotSampled with better descriptive parentBased (under sampling preview)

Side effect:

• You can now specify higher sampling percentage to downstream spans, e.g. you can say that you want to capture 100% of a specific dependency, even if that is part of a trace that is not sampled-in. Can remove the warning note at https://docs.microsoft.com/en-us/azure/azure-monitor/app/java-standalone-sampling-overrides#how-it-works.

This behavior change around parent-based sampling probably deserves a minor version bump (to 3.4.0) and we should post follow-ups to #2152 and #1883 where the ignoreRemoteParentNotSampled was previously discussed.