MapColonies / shigola / 456db4385

57%

master: 44%

push

github

feat(log): put trace and span ids on records inside a trace (MAPCO-11494) (#23)

* feat(log): put trace and span ids on records inside a trace

Every log line written while serving a traced request now carries
trace_id and span_id, so a trace in Tempo reaches the log lines it
produced and a log line reaches its trace.

The ids come from the span context on the context slog.Handler.Handle is
already handed, which is why this is cheap: no call site has to be
taught to pass them, and a record outside a trace yields the zero span
context and so gains nothing. Sampling is deliberately not consulted —
at the default 1% ratio most requests are unsampled, and dropping their
ids would leave 99% of requests' log lines uncorrelated with each other,
which is the cheaper half of what this is for.

Two supporting pieces:

ServiceAttrs replaces the WithGroup("shigola") the binaries used. An
open group qualifies everything after it, including the attributes
Handle adds per record, so correlation would have been logged as
shigola.trace_id — not a name a log pipeline looks for. As one grouped
attribute the group nests only its own contents, so what it writes is
byte-for-byte what WithGroup wrote; the one visible change is that the
stack trace on an error record is now top-level too. It returns an
slog.Attr rather than a logger because an slog.Attr can only reach a
logger through With, so the placement cannot be undone by a caller
reaching for WithGroup out of habit.

The *fContext helpers are the migration path for the request path's
existing printf-style call sites: they let a site gain correlation
without its message changing, so nothing that greps today's logs has to
be re-taught for the sake of two fields. They go the same way as their
context-free siblings when that TODO is done.

Costs nothing when tracing is off, and is not gated on the config:
BenchmarkHandleWithoutATrace is within run noise of the same handler
without the context lookup, at zero allocations.

... (continued)

110 of 151 new or added lines in 12 files covered. (72.85%)

5810 of 10438 relevant lines covered (55.66%)

122.86 hits per line