feat(metrics): Migrate to OpenTelemetry for metrics

This commit migrates the metrics collection from OpenCensus to OpenTelemetry, aligning with the broader observability trends in the ecosystem.

Key changes include:

- Updated `pipelinerunmetrics` and `taskrunmetrics` to use the OpenTelemetry SDK for creating and recording metrics.
- Preserved existing metric names and labels to ensure backward compatibility with existing dashboards and alerting.
- Updated `config-observability.yaml` to use the new OpenTelemetry configuration for exporting metrics.
- Updated `docs/metrics.md` to reflect the new configuration options.
- Removed the now-obsolete OpenCensus `unregisterMetrics` function from the test suite.

All changes have been verified with the existing test suite.

Author: khrm

config/config-observability.yaml

@@ -37,6 +37,43 @@ data:
     # this example block and unindented to be in the data block
     # to actually change the configuration.
 
+    # OpenTelemetry Metrics Configuration
+    # Protocol for metrics export (prometheus, grpc, http/protobuf, none)
+    # Default: prometheus
+    metrics-protocol: prometheus
+    
+    # Metrics endpoint (for grpc/http protocols)
+    # Default: empty (uses default OTLP endpoint)
+    metrics-endpoint: ""
+    
+    # Metrics export interval (e.g., "30s", "1m")
+    # Default: empty (uses default interval)
+    metrics-export-interval: ""
+
+    # OpenTelemetry Tracing Configuration
+    # Protocol for tracing export (grpc, http/protobuf, none, stdout)
+    # Default: none
+    tracing-protocol: none
+    
+    # Tracing endpoint (for grpc/http protocols)
+    # Default: empty
+    tracing-endpoint: ""
+    
+    # Tracing sampling rate (0.0 to 1.0)
+    # Default: 1.0 (100% sampling)
+    tracing-sampling-rate: "1.0"
+
+    # Runtime Configuration
+    # Enable profiling (enabled, disabled)
+    # Default: disabled
+    runtime-profiling: disabled
+    
+    # Runtime export interval (e.g., "15s")
+    # Default: 15s
+    runtime-export-interval: "15s"
+
+    # Legacy OpenCensus Configuration (DEPRECATED)
+    # These are kept for backward compatibility but should be migrated to OpenTelemetry
     # metrics.backend-destination field specifies the system metrics destination.
     # It supports either prometheus (the default) or stackdriver.
     # Note: Using Stackdriver will incur additional charges.
@@ -54,9 +91,11 @@ data:
     # charge.  If metrics.backend-destination is not Stackdriver, this is
     # ignored.
     metrics.allow-stackdriver-custom-metrics: "false"
+    
+    # Tekton-specific metrics configuration
     metrics.taskrun.level: "task"
     metrics.taskrun.duration-type: "histogram"
     metrics.pipelinerun.level: "pipeline"
     metrics.pipelinerun.duration-type: "histogram"
     metrics.count.enable-reason: "false"
-    metrics.running-pipelinerun.level: ""
+    metrics.running-pipelinerun.level: ""