feat: Add Attempt, Operation and GFE Metrics

[lszinv]

Implementation of the MetricTracer and SpannerMetricFactory, a specific instance of the MetricFactory that populates the factory with metadata specific to the Python Spanner Client.

The Implemented metrics:

Metric Name	Description
Attempt Latency	latency of an RPC attempt.
Attempt Count	The count of RPC attempts
Operation Latency	The total end-to-end latency across all request attempts, including retries.
Operation Count	The count of operations
GFE Latency	The latency of GFE
GFE Header missing count	The count of RPCs where GFE header is missing

[rahul2393]

As per offline discussion waiting for unit test to mock RPC failure and validate attempt_count>1

[rahul2393]

@lszinv Build is failing

INFO: pip is looking at multiple versions of google-cloud-spanner[tracing] to determine which version is compatible with other requirements. This could take a while.
ERROR: Ignored the following versions that require a different python version: 0.44b0 Requires-Python >=3.8; 0.45b0 Requires-Python >=3.8; 0.46b0 Requires-Python >=3.8; 0.47b0 Requires-Python >=3.8; 0.48b0 Requires-Python >=3.8; 0.49b0 Requires-Python >=3.8; 0.49b1 Requires-Python >=3.8; 0.49b2 Requires-Python >=3.8; 0.50b0 Requires-Python >=3.8; 1.23.0 Requires-Python >=3.8; 1.24.0 Requires-Python >=3.8; 1.25.0 Requires-Python >=3.8; 1.26.0 Requires-Python >=3.8; 1.27.0 Requires-Python >=3.8; 1.28.0 Requires-Python >=3.8; 1.28.1 Requires-Python >=3.8; 1.28.2 Requires-Python >=3.8; 1.29.0 Requires-Python >=3.8; 5.0.0 Requires-Python >=3.8; 5.0.1 Requires-Python >=3.8; 5.1.0 Requires-Python >=3.9
ERROR: Could not find a version that satisfies the requirement mmh3>=5.0.1 (from google-cloud-spanner[tracing]) (from versions: 2.0, 2.2, 2.3, 2.3.1, 2.4, 2.5, 2.5.1, 3.0.0, 3.1.0, 4.0.0, 4.0.1, 4.1.0)
ERROR: No matching distribution found for mmh3>=5.0.1
nox > Session unit-3.7(protobuf_implementation='python') failed.

[lszinv]

• Rebased onto up-to-date main
• updated the minimum version to address build failure
• Added unit test to verify attempt count metrics increasing on failure
• Verified that Periodic Exporter is defaulted to export at 1 minute intervals, but explicitly set the interval too to guard against future changes to the default value.

[lszinv]

Investigating the 3.7 test failure.

[lszinv]

Ignore the last commit. Will be pushing a new one to fix the test without disabling it.

[lszinv]

Apologies, forgot to run the mockserver tests with the last change. I've fixed the test failures.