refactor: optimize Spanner batching and standardize OpenTelemetry deployment

[jcscottiii]

This commit optimizes Spanner write performance and standardizes OpenTelemetry (OTel) deployment across the backend API, background workers, and ingestion jobs.

1. Spanner Batching:

• Refactored UpsertWPTRunFeatureMetrics in lib/gcpspanner/wpt_run_feature_metric.go to execute two batch queries at the transaction start instead of executing queries inside a loop, reducing database roundtrips.
• Extracted mutation building to buildWPTRunFeatureMetricMutations to reduce cognitive complexity.

2. Centralized Terraform Telemetry Configuration:

• Created root-level infra/telemetry.tf to define a single shared Secret Manager secret containing the OTel collector configuration.
• Swapped the custom in-tree OTel collector image for the official Google-managed image (otelcol-google:0.151.0). This is to match this new documentation: https://docs.cloud.google.com/stackdriver/docs/instrumentation/opentelemetry-collector-cloud-run
• Defined a centralized otel_collector_config_mount_path local variable in infra/telemetry.tf set to "/etc/otelcol-google" and propagated it to all submodules, replacing hardcoded paths.

3. OTel Sidecar Deployment (Go & Terraform):

• Added opentelemetry.MaybeSetup helper to lib/opentelemetry/setup.go to encapsulate environment checks and OTel SDK initialization.
• Refactored backend/cmd/server/main.go and all 11 background workers and daily ingestion scraper entrypoints to call MaybeSetup and defer shutdown.
• Deployed the OTel sidecar container and mounted the shared config secret across all 4 worker pools and the reusable job module.
• Granted roles/cloudtrace.agent, roles/monitoring.metricWriter, and roles/logging.logWriter to the backend and worker service accounts.

4. GCP Error Reporting & Structured Logging:

• Updated the custom slog handler in lib/opentelemetry/slog.go to capture and append runtime/debug.Stack() to ERROR logs.
• Structured logs to enable automatic GCP Error Reporting aggregation and trace-log linking using the trace field.

5. Go Startup Logging & Refactoring:

• Added verbose BOOT: log statements before each client initialization phase (Datastore, Spanner, Valkey, OTel) in backend/cmd/server/main.go to provide startup phase visibility.
• Refactored the inline OpenTelemetry setup block in main.go to use the new opentelemetry.MaybeSetup helper, keeping the startup sequence synchronous.

6. Repository Cleanup:

• Deleted the unused custom in-tree OTel collector Dockerfile (otel/Dockerfile).
• Removed the /otel Docker package-ecosystem update entry from .github/dependabot.yml.

BUG=526562255