chore: Add Custom OpenTelemetry Exporter in for Service Metrics

[lszinv]

This change adds an implementation for an OpenTelemetry exporter to send the following service metrics that are already present in the Go and Java clients:

• operation_latencies
• operation_count
• attempt_latencies
• attempt_count
• gfe_latency
• gfe_missing_header_count

Files added to a metrics folder in /google/cloud/spanner_v1/metrics

• metrics_exporter.py - The definition for the exporter
• constants.py - Constant values such as metric and label names to be used by both the exporter and eventual implementing code.

Based on regular client Google Cloud Monitoring exporter found here

note: CLA not signed yet, as there's a settings issue. Will have it submitted soon.
note2: Linter runs with no issues, currently working on generating code coverage report to verify full coverage.

[google-cla]

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