Set TraceOperation to sampled=true by default

[marcotc]

Fixes #2101

Motivation

In v0.x of ddtrace, the currently active trace, represented by the Context class, would always have a sampled == true by default, as it inherited its value from Span#sampled:

dd-trace-rb/lib/ddtrace/context.rb

Line 299 in 9fd0444

@sampled = span.sampled

And Span#sampled defaults to true:

dd-trace-rb/lib/ddtrace/span.rb

Line 94 in 9fd0444

@sampled = true

In v1.x, the currently active trace is now represented by a TraceOperation.
TraceOperation#sampled defaults to false:

dd-trace-rb/lib/datadog/tracing/trace_operation.rb

Line 77 in 4f646ea

@sampled = sampled.nil? ? false : sampled

And there's no mechanism to inherit sampling from the active span in v1.x.

This doesn't normally affect traces, as the PrioritySampler, which is the default sampler, always sets sampled = true:

dd-trace-rb/lib/datadog/tracing/sampling/priority_sampler.rb

Lines 92 to 98 in 4f646ea

	# NOTE: We'll want to leave `trace.sampled = true` here; all spans for priority sampling must
	# be sent to the agent. Otherwise metrics for traces will not be accurate, since the
	# agent will have an incomplete dataset.
	#
	# We also ensure that the agent knows we that our `post_sampler` is not performing true sampling,
	# to avoid erroneous metric upscaling.
	trace.sampled = true

But, sampling checks are completely skipped if the sampling decision was made in an upstream service. We only check the sampler if the current span is a root span (parent_id == 0):

dd-trace-rb/lib/datadog/tracing/tracer.rb

Line 345 in 4f646ea

sample_trace(event_trace_op) if event_span_op && event_span_op.parent_id == 0

Because distributed traces won't have any spans with parent_id == 0, as the local root span inherits the parent_id from the upstream distributed span, we never invoked the PrioritySampler.

What does this PR do?

This PR sets TraceOperation#sampled to true by default, as having traces being sampled by default was the desired intent in first place.