[WIP] Common Expression Language (CEL) sampler

[dol]

This pull request introduces a new CelBasedSampler to the OpenTelemetry Java SDK, enabling advanced sampling rules using the Common Expression Language (CEL). It also includes updates to the existing RuleBasedRoutingSampler for consistency and clarity. Below is a summary of the most important changes grouped by theme:

New CelBasedSampler Implementation:

• Added the CelBasedSampler class, which evaluates CEL expressions to make sampling decisions based on span attributes. It includes a fallback sampler and supports multiple expressions. (samplers/src/main/java/io/opentelemetry/contrib/sampler/CelBasedSampler.java)
• Introduced the CelBasedSamplerBuilder to construct CelBasedSampler instances with methods for adding expressions and specifying actions (e.g., DROP or RECORD_AND_SAMPLE). (samplers/src/main/java/io/opentelemetry/contrib/sampler/CelBasedSamplerBuilder.java)
• Added the CelBasedSamplingExpression class to encapsulate individual CEL expressions and their associated samplers. (samplers/src/main/java/io/opentelemetry/contrib/sampler/CelBasedSamplingExpression.java)
• Implemented a declarative configuration provider for CelBasedSampler, enabling configuration through YAML files. (samplers/src/main/java/io/opentelemetry/contrib/sampler/internal/CelBasedSamplerComponentProvider.java)

# The fallback sampler to use if no expressions match.
fallback_sampler:
  always_on:
# List of CEL expressions to evaluate. Expressions are evaluated in order.
expressions:
  # The action to take when the expression evaluates to true. Must be one of: DROP, RECORD_AND_SAMPLE.
  - action: DROP
    # The CEL expression to evaluate. Must return a boolean.
    expression: attribute['url.path'].startsWith('/actuator')
  - action: RECORD_AND_SAMPLE
    expression: attribute['http.method'] == 'GET' && attribute['http.status_code'] < 400

Documentation Updates:

• Updated samplers/README.md to document the new CelBasedSampler, including its usage, schema, and example configurations. (samplers/README.md) [1] [2]

Dependency Additions:

• Added a dependency on the dev.cel:cel:0.9.0 library to enable CEL expression evaluation. (samplers/build.gradle.kts)

Updates to RuleBasedRoutingSampler:

• Renamed SamplingRule to RuleBasedRoutingSamplingRule for clarity and updated all related references in the RuleBasedRoutingSampler and its builder. (samplers/src/main/java/io/opentelemetry/contrib/sampler/RuleBasedRoutingSampler.java, samplers/src/main/java/io/opentelemetry/contrib/sampler/RuleBasedRoutingSamplerBuilder.java, samplers/src/main/java/io/opentelemetry/contrib/sampler/RuleBasedRoutingSamplingRule.java) [1] [2] [3] [4] [5] [6] [7]

These changes collectively enhance the SDK's sampling capabilities, allowing users to define sophisticated sampling rules using CEL while maintaining compatibility with existing samplers.