OpenTelemetry logging support

[gnurizen]

Summary

Adds OTLP/gRPC logs support to parca-agent so any in-process producer
(the eBPF probes service in PR #3181, the agent's own logrus output via
the new --otlp-logging flag) can ship structured log records over the
same gRPC connection already used for profile data.

Built on the upstream OTel Go SDK (sdklog.LoggerProvider +
otlploggrpc.Exporter), so retry, backoff, queue, batch, and
PartialSuccess handling come from the SDK rather than being
hand-rolled.

Commits

• reporter: add ParcaReporter log-event interface and per-sample relabel

  • New ParcaReporter interface extending OTel's TraceReporter with
    Logger(scope string) log.Logger. Producers obtain a per-scope
    log.Logger and emit records directly.
  • Reporter struct renamed arrowReporter and now owns a
    *sdklog.LoggerProvider. The provider uses otlploggrpc.New(... WithGRPCConn(conn)) so it shares the agent's existing gRPC
    connection.
  • sdklog.NewBatchProcessor configured with size 512 / age 250 ms /
    queue depth 4096 — tighter than the SDK default so individual events
    land on the server within a few hundred ms.
  • sdklog.WithResource fills service.name, service.version,
    host.name as required attributes.
  • Offline mode (no gRPC conn) returns the OTel no-op logger; emit
    calls become inert and producers don't need an extra nil check.
  • Also: per-sample relabel pass for probe-origin trace samples
    (labelsForTID runs a second relabel.ProcessBuilder against
    patched per-sample labels when meta.Origin == TraceOriginProbe).

• reporter: --otlp-logging flag forwards agent logs over OTLP

  • New --otlp-logging flag, default off. When set, installs a logrus
    hook that converts each entry to an OTel log.Record and emits via
    Logger(\"parca-agent.agent\").
  • Severity / SeverityText come from the logrus level; a level
    attribute is always emitted so consumers can filter by level even
    though the OTLP server doesn't store severity_text directly.
  • OTLPSkipField ("otlp_skip") is an opt-out: entries tagged with
    this field set to true are dropped before emit. Used by the probes
    service's per-fire debug logging to avoid double-shipping.
  • In offline mode the flag logs a one-shot warning and behaves as
    off, since Logger() returns the no-op variant.

gRPC codec extension

flags/codec.go gains handling for pdata's SizeProto/MarshalProto
shape so the global vtproto codec doesn't reject the pdata-typed
requests otlploggrpc sends. No change in behavior for existing
profile-data paths (which use vtproto or gogoproto).

Test plan

• `go build ./...` clean
• `go test ./reporter/... -count=1` — all pass; new unit tests
  cover the logrus hook (severity / attribute / skip-field / level
  mapping) using an in-process `captureExporter` + SDK
  `SimpleProcessor`
• Local end-to-end run on linux/amd64 against PolarSignals backend:
  agent logs flow as OTLP records under `attributes_resource.host.name`
• Reviewer: confirm gRPC connection is correctly shared with the
  profile-data path (no second TCP/TLS handshake) — `WithGRPCConn`
  should make this trivial to verify with `ss -tnp`

Stacked on

PR #3181 (`simple-probes-v1`) rebases on top of this branch and adds
the eBPF probes service as a third Logger consumer.