chore: Add Custom OpenTelemetry Exporter in for Service Metrics

[ravjotbrar]

Description

This change adds an implementation for an OpenTelemetry exporter to send the following service metrics that are already present in the Python, 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 /src/metrics/ folder:

constants.ts - Constant values such as metric and label names to be used by both the exporter and eventual implementing code.
spanner-metrics-exporter.ts - The definition for the exporter
external-types.ts - Type definition for ExporterOptions, MetricKind, and ValueType
transform.ts - Methods that help create a GCM timeseries list from scope metrics.

Based on regular client Google Cloud Monitoring exporter found here

Impact

Integrates Built-in Metrics support into Node.js spanner client library.

Testing

Unit tests were added in /test/metrics

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added, particularly in complex areas or places that require background
• No new warnings or issues will be generated from this change

Fixes #issue_number_goes_here 🦕

[lszinv]

Looking to resolve the CI failures on older node versions.

Will see if I can reduce usage of the new packages

[lszinv]

Please close this as edits/changes are being handled in #2272 instead