OpenTelemetry:Fix attribute merge with Datadog tags

[marcotc]

What does this PR do?
This PR fixes an issue with the tracing OpenTelemetry integration where OpenTelemetry span attributes were processed and converted to Datadog span tags, but not correctly converted back into a Hash object; they were left as an Array of key/value pairs.
Because an Array of key/value pairs is not what Tracing.trace(tags:) expects, this should have failed much earlier. But it actually has a chance of successfully setting span tags in the scenario that:

1. There are no global DD_TAGS. In this case, we don't need to merge the existing tag array with the provided Array of key/value pairs.

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

   Line 423 in 7ca3042

   tags || @tags.dup

2. set_tags iterates over the provided tags in a way that works for both Hashes and Arrays of key/value pairs:

   dd-trace-rb/lib/datadog/tracing/metadata/tagging.rb

   Line 65 in 7ca3042

   hash.each { |k, v| set_tag(k, v) }

This means that unlessDD_TAGS (or the equivalent c.tags = ...) was set, everything worked as expected.

Motivation:
To fix this error:

lib/datadog/tracing/tracer.rb:429:in `merge': no implicit conversion of Array into Hash (TypeError)	
lib/datadog/tracing/tracer.rb:429:in `resolve_tags'	
lib/datadog/tracing/tracer.rb:404:in `start_span'	
lib/datadog/tracing/tracer.rb:176:in `trace'	
lib/datadog/tracing.rb:18:in `trace'	
lib/datadog/opentelemetry/sdk/span_processor.rb:85:in `start_datadog_span'

How to test the change?
We have unit tests for this change.

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