Add Trace Generation Metadata

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Running generator for google/devtools/cloudtrace/artman_cloudtrace_v2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-trace-v2.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/TraceServiceGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/TraceServiceClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/TraceServiceClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V2/TraceServiceSmokeTest.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_Attributes.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_TimeEvent_MessageEvent_Type.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_TimeEvent_Annotation.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_Links.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_Link.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/StackTrace_StackFrame.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_TimeEvents.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_Link_Type.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/StackTrace_StackFrames.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_TimeEvent_MessageEvent.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Span_TimeEvent.php.
synthtool > Replaced '@type Span_' in src/V2/Gapic/TraceServiceGapicClient.php.
synthtool > Replaced 'Span_' in src/V2/Gapic/TraceServiceGapicClient.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.