Expose measured duration on Timer context manager

[vokracko]

Motivation

When you use .time() as a context manager, the duration is already computed internally but thrown away — if you want to log or reuse it, you end up measuring twice:

start = time.monotonic()
with REQUEST_LATENCY.time():
    response = handle_request()
logger.info("request took %.3fs", time.monotonic() - start)

That's redundant and slightly off (the two measurements don't cover the same interval). This change stores the observed value on the Timer so it can be read back after the block exits.

Usage

with REQUEST_LATENCY.time() as t:
    response = handle_request()

logger.info("request took %.3fs", t.duration)

t.duration is None before the block runs and holds the observed seconds (the same value passed to observe() / set()) afterwards.

Compatibility

Purely additive. Existing with metric.time(): usage is unchanged — the attribute is only visible if you bind the context manager with as.

[csmarchbanks]

One small suggestion, but 👍, nice little quality of life improvement!

[csmarchbanks]

Thanks!