feat: (observability, samples): add tracing end-to-end sample

[odeke-em]

This change documents an end-to-end observability tracing sample using OpenTelemetry, which then exports trace spans to Google Cloud Trace.

Updates #2079

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml
• samples/README.md - samples/README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

[snippet-bot[bot]]

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[odeke-em]

@surbhigarg92 @alkatrivedi @harshachinta kindly help me run the bots on this code, thank you!

[odeke-em]

Kindly help me run the bots @surbhigarg92! Thank you.

[alkatrivedi]

New samples should follow rubric format.

Refer: https://github.com/googleapis/nodejs-spanner/blob/main/samples/delete-backup-schedule.js

[odeke-em]

@alkatrivedi observability includes traces and metrics. That rubric is behind Google Corp and my email isn't activated.

[odeke-em]

@alkatrivedi sure let me tweak it look like that sample, thanks!

[odeke-em]

@alkatrivedi kindly please take a look again, thank you!

[alkatrivedi]

@odeke-em please add a test for the sample in https://github.com/googleapis/nodejs-spanner/blob/main/samples/system-test/spanner.test.js

[odeke-em]

Done, thank you @alkatrivedi! Please take another look.

[surbhigarg92]

@odeke-em Please also work on the github lint issues.