Gate OpenTelemetry attribute building on the sampling decision

[nhachicha]

Reduces the Java driver's OpenTelemetry tracing overhead, following the Implementation Guidelines in mongodb/specifications#1986 (DRIVERS-3620). Draft: needs a JAVA ticket and a decision on the reflective probe before review proper.

Why

Benchmarking on the spec's method showed Java paying almost the same cost with a 1% sampler as with every span recorded. A JFR profile under sdk-ratio put 34% of CPU samples on tracing frames, in this order:

1. Building the tag set on span stop. Micrometer calls DefaultMongodbObservationConvention for all low- and high-cardinality key values unconditionally in Observation.stop(), and the convention chained KeyValues.and(), which merges and re-sorts per call. This ran on every command whether or not the sampler kept the span.
2. TracingManager.createTracingSpan decoded the just-encoded BSON command into a ByteBufBsonDocument only to read its first key.
3. Scope thread-local handling inside the OTel bridge (not touched).

What changes

• MongodbObservationContext: new recording flag (default true).
• MicrometerTracer: after observation.start(), asks TracingSpanRecordingProbe whether the SDK is recording the span; clears the flag if not.
• TracingSpanRecordingProbe (new, internal): driver-core depends only on micrometer-observation, so the micrometer-tracing span type is resolved reflectively once via method handles (TracingObservationHandler.TracingContext.getSpan(), Span.isNoop()). Micrometer's tracing handler stores the span in the context under that class key during onStart, inside start(); the OTel bridge's isNoop() is !isRecording(). When micrometer-tracing is absent or no span is attached, every span is treated as recording (previous behaviour).
• DefaultMongodbObservationConvention: high-cardinality key values are skipped for a non-recording span; all three builders fill one KeyValue[] and call KeyValues.of once. Low-cardinality values are still produced for non-recording spans because metrics handlers on the same registry consume them.
• CommandMessage.getCommandName(); TracingManager uses it and decodes the command only for getMore (cursor id).

No public API change beyond two accessors on MongodbObservationContext. Tag names and values are unchanged.

Verification

• New DefaultMongodbObservationConventionTest (6) and a CommandMessageTest case: pass.
• MicrometerProseTest sync and reactive: pass.
• Unified MicrometerTracingTest sync and reactive at the pinned spec revision: pass except the two tests needing configureFailPoint, which the local server lacked.
• Span counts unchanged in verify mode (2/op sdk-always, 0.02/op sdk-ratio).

Measured effect (same host and harness, 5 interleaved reps, medians; loss vs off / added CPU µs per op excl. GC)

API	Task	Config	Before	After
sync	Small doc insertOne	api-only	+4.2% / +1.6	+1.0% / +0.2
sync	Small doc insertOne	sdk-ratio	+9.6% / +4.5	+6.2% / +2.2
sync	Small doc insertOne	sdk-always	+10.8% / +5.3	+7.9% / +3.0
sync	Find one by ID	api-only	+5.4% / +2.2	+0.9% / +0.2
sync	Find one by ID	sdk-ratio	+12.2% / +5.6	+5.3% / +2.3
sync	Find one by ID	sdk-always	+13.3% / +6.7	+7.1% / +3.2
async	Small doc insertOne	api-only	+3.1% / +1.7	+0.6% / +0.4
async	Small doc insertOne	sdk-ratio	+5.5% / +4.3	+1.4% / +1.1
async	Small doc insertOne	sdk-always	+6.3% / +5.5	+2.6% / +1.4
async	Find one by ID	api-only	+2.6% / +0.7	−0.2% / +0.0
async	Find one by ID	sdk-ratio	+6.3% / +4.5	+2.3% / +1.7
async	Find one by ID	sdk-always	+7.5% / +5.5	+5.0% / +3.0

Added CPU per op roughly halved in every configuration; api-only is within 1% of off on all four tasks. The sdk-ratio/sdk-always gap stays small because the remaining cost is span creation and scope handling in the SDK, which bears on the spec PR's gating diagnostic.

Harness: companion draft "DRIVERS-3620: OpenTelemetry overhead benchmark suite".

Review notes

• The reflective probe is the part I would most like eyes on. The alternative is an optionalImplementation on micrometer-tracing, which is heavier for one boolean.
• Needs a JAVA ticket; branch will be renamed to match.