message: fix(tracing): Make `method` required in `transactionSampling` type
  Though every transaction should have `transactionSampling` metadata, we mark the property as optional because we add it after the transaction is created, in the process of computing a sampling decision for it. Once `transactionSampling` exists, however, it should always have a `method` property.
diff: @@ -120,7 +120,7 @@ export type Measurements = Record<string, { value: number }>;
  export type TransactionSamplingMethod = 'explicitly_set' | 'client_sampler' | 'client_rate' | 'inheritance';
  export interface TransactionMetadata {
  - transactionSampling?: { rate?: number; method?: TransactionSamplingMethod };
  + transactionSampling?: { rate?: number; method: TransactionSamplingMethod };
  /** The two halves (sentry and third-party) of a transaction's tracestate header, used for dynamic sampling */
  tracestate?: {
  