Add example of prometheus exemplars being scraped from a flask app

[aabmass]

See README.rst and screenshots for more explanation. This is a decent way to get exemplars until OpenTelemetry Python implements them.

Preview: https://google-cloud-opentelemetry--245.org.readthedocs.build/en/245/examples/prometheus_exemplars/README.html

[snippet-bot]

Here is the summary of changes.

You are about to add 7 region tags.

• docs/examples/prometheus_exemplars/README.rst:58, tag opentelemetry_prom_exemplars_attach
• docs/examples/prometheus_exemplars/README.rst:68, tag opentelemetry_prom_exemplars_observe
• docs/examples/prometheus_exemplars/server.py:16, tag opentelemetry_prom_exemplars_import
• docs/examples/prometheus_exemplars/server.py:38, tag opentelemetry_prom_exemplars_setup_exporter
• docs/examples/prometheus_exemplars/server.py:63, tag opentelemetry_prom_exemplars_instrument
• docs/examples/prometheus_exemplars/server.py:94, tag opentelemetry_prom_exemplars_observe
• docs/examples/prometheus_exemplars/server.py:104, tag opentelemetry_prom_exemplars_attach

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