Support OTEL_SDK_DISABLED

[pbrisbin]

We'd love to see support for OTEL_SDK_DISABLED, so we don't need conditionals in our code to only enable tracing in environments where it's appropriate. Today, we add code to read this variable ourselves and initialize with emptyTracerProviderOptions when set, or our usual way when not.

https://opentelemetry.io/docs/reference/specification/sdk-environment-variables/#general-sdk-configuration

Boolean value. If “true”, a no-op SDK implementation will be used for all telemetry signals. Any other value or absence of the variable will have no effect and the SDK will remain enabled. This setting has no effect on propagators configured through the OTEL_PROPAGATORS variable.

Further details on encoding booleans: https://opentelemetry.io/docs/reference/specification/sdk-environment-variables/#boolean-value.

Any value that represents a Boolean MUST be set to true only by the case-insensitive string "true", meaning "True" or "TRUE" are also accepted, as true. An SDK MUST NOT extend this definition and define additional values that are interpreted as true. Any value not explicitly defined here as a true value, including unset and empty values, MUST be interpreted as false. If any value other than a true value, case-insensitive string "false", empty, or unset is used, a warning SHOULD be logged to inform users about the fallback to false being applied. All Boolean environment variables SHOULD be named and defined such that false is the expected safe default behavior. Renaming or changing the default value MUST NOT happen without a major version upgrade.