Gracefully degrade on metrics encoding failures#213
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By default, `prometheus-client` aborts metrics collection/encoding on _any_ error. This means one error from the EncodeMetric implementation of a single metric in a Registry makes all the metrics in that Registry unavailable. To avoid this, we introduce an EncodeMetric wrapper that swallows errors (after reporting them via logging/`eprintln!`.) Due to the way `prometheus-client` is designed, this could leave a partially-written metrics line in the output. We fix this by introducing `RewindableWriter`, a `Write` implementation that we can rewind to the last newline in case we do see an error. `prometheus-client` does not give us access to the underlying writer in EncodeMetric impls, so we are forced to use a side channel via thread-local storage to activate the rewind behavior.
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By default,
prometheus-clientaborts metrics collection/encoding on any error. This means one error from the EncodeMetric implementation of a single metric in a Registry makes all the metrics in that Registry unavailable.To avoid this, we introduce an EncodeMetric wrapper that swallows errors (after reporting them via logging/
eprintln!.) Due to the wayprometheus-clientis designed, this could leave a partially-written metrics line in the output. We fix this by introducingRewindableWriter, aWriteimplementation that we can rewind to the last newline in case we do see an error.prometheus-clientdoes not give us access to the underlying writer in EncodeMetric impls, so we are forced to use a side channel via thread-local storage to activate the rewind behavior.