perf(tracer): avoid unncessary context creation (#14773)

## Description

Currently for a new root span we are creating 2 context objects. We
create an empty one in `Tracer._start_span`, and then it gets copied
when we create the root span.

This is unnecessary, we can avoid this extra object allocation/GC.

Benchmarks showing about a 12-15% reduction in root span creation
overhead (only for starting the span, the span-finish scenario is 4-5%
faster).

## Testing

<!-- Describe your testing strategy or note what tests are included -->

## Risks

<!-- Note any risks associated with this change, or "None" if no risks
-->

## Additional Notes

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Author: brettlangdon

.gitlab/benchmarks/bp-runner.microbenchmarks.fail-on-breach.yml

@@ -799,11 +799,11 @@ experiments:
           - name: otelspan-add-metrics
             thresholds:
               - execution_time < 344.80 ms
-              - max_rss_usage < 562.00 MB
+              - max_rss_usage < 600.00 MB
           - name: otelspan-add-tags
             thresholds:
               - execution_time < 314.00 ms
-              - max_rss_usage < 563.50 MB
+              - max_rss_usage < 600.00 MB
           - name: otelspan-get-context
             thresholds:
               - execution_time < 92.35 ms
@@ -1027,55 +1027,55 @@ experiments:
           # span
           - name: span-add-event
             thresholds:
-              - execution_time < 26.20 ms
+              - execution_time < 22.50 ms
               - max_rss_usage < 53.00 MB
           - name: span-add-metrics
             thresholds:
-              - execution_time < 98.35 ms
+              - execution_time < 93.50 ms
               - max_rss_usage < 961.00 MB
           - name: span-add-tags
             thresholds:
-              - execution_time < 168.55 ms
+              - execution_time < 155.00 ms
               - max_rss_usage < 962.50 MB
           - name: span-get-context
             thresholds:
-              - execution_time < 23.70 ms
+              - execution_time < 20.50 ms
               - max_rss_usage < 53.00 MB
           - name: span-is-recording
             thresholds:
-              - execution_time < 23.90 ms
+              - execution_time < 20.50 ms
               - max_rss_usage < 53.00 MB
           - name: span-record-exception
             thresholds:
-              - execution_time < 44.50 ms
+              - execution_time < 40.00 ms
               - max_rss_usage < 53.00 MB
           - name: span-set-status
             thresholds:
-              - execution_time < 26.00 ms
+              - execution_time < 22.00 ms
               - max_rss_usage < 53.00 MB
           - name: span-start
             thresholds:
-              - execution_time < 23.50 ms
+              - execution_time < 20.50 ms
               - max_rss_usage < 53.00 MB
           - name: span-start-finish
             thresholds:
-              - execution_time < 55.50 ms
+              - execution_time < 52.50 ms
               - max_rss_usage < 34.00 MB
           - name: span-start-finish-telemetry
             thresholds:
-              - execution_time < 58.30 ms
+              - execution_time < 54.50 ms
               - max_rss_usage < 34.00 MB
           - name: span-start-finish-traceid128
             thresholds:
-              - execution_time < 60.05 ms
+              - execution_time < 55.00 ms
               - max_rss_usage < 34.00 MB
           - name: span-start-traceid128
             thresholds:
-              - execution_time < 24.60 ms
+              - execution_time < 22.50 ms
               - max_rss_usage < 53.00 MB
           - name: span-update-name
             thresholds:
-              - execution_time < 24.10 ms
+              - execution_time < 22.00 ms
               - max_rss_usage < 53.00 MB
 
           # telemetryaddmetric

ddtrace/_trace/tracer.py

@@ -466,17 +466,19 @@ def _start_span(
                 child_of = new_ctx
 
         parent: Optional[Span] = None
+        context: Optional[Context] = None
+        trace_id: Optional[int] = None
+        parent_id: Optional[int] = None
+
         if child_of is not None:
             if isinstance(child_of, Context):
                 context = child_of
             else:
                 context = child_of.context
                 parent = child_of
-        else:
-            context = Context(is_remote=False)
 
-        trace_id = context.trace_id
-        parent_id = context.span_id
+            trace_id = context.trace_id
+            parent_id = context.span_id
 
         # The following precedence is used for a new span's service:
         # 1. Explicitly provided service name
@@ -493,8 +495,8 @@ def _start_span(
         # Update the service name based on any mapping
         service = config.service_mapping.get(service, service)
 
-        links = context._span_links if not parent else []
-        if trace_id or links or context._baggage:
+        links = context._span_links if not parent and context else []
+        if trace_id or links or (context and context._baggage):
             # child_of a non-empty context, so either a local child span or from a remote context
             span = Span(
                 name=name,