Within Google's Site Reliability Engineering framework, which concept measures how well a system is performing?

A Service Level Agreement (SLA) is a contractual commitment between a provider and customer that defines consequences if service targets are not met - it is a business contract, not a performance measurement metric.

A Service Level Indicator (SLI) is a carefully defined quantitative measure of a specific aspect of the service being provided - for example, request latency, availability percentage, or error rate. In the Google SRE framework, SLIs are the raw performance measurements that capture how well a system is actually performing at any given point in time, making them the direct answer to the question of what measures system performance.

Error reporting is a diagnostic tool for capturing, grouping, and alerting on application errors, but it is not the SRE framework concept that holistically measures overall system performance.

A Service Level Objective (SLO) is a target threshold or goal set for an SLI metric - it defines the desired level of performance to achieve, not the actual measurement of current system performance.