Reuse convertValue in otelslog.

Follows
#6237.

### Added

- Add the `WithSource` option to the
`go.opentelemetry.io/contrib/bridges/otelslog` log bridge to set the
`code.*` attributes in the log record that includes the source location
where the record was emitted. (#6253)
- Add `ContextWithStartTime` and `StartTimeFromContext` to
`go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp`, which
allows setting the start time using go context. (#6137)
- Set the `code.*` attributes in
`go.opentelemetry.io/contrib/bridges/otelzap` if the `zap.Logger` was
created with the `AddCaller` or `AddStacktrace` option. (#6268)
- Add a `LogProcessor` to
`go.opentelemetry.io/contrib/processors/baggagecopy` to copy baggage
members to log records. (#6277)
  - Use `baggagecopy.NewLogProcessor` when configuring a Log Provider.
- `NewLogProcessor` accepts a `Filter` function type that selects which
baggage members are added to the log record.

### Changed 

- Transform raw (`slog.KindAny`) attribute values to matching
`log.Value` types.
For example, `[]string{"foo", "bar"}` attribute value is now transformed
to `log.SliceValue(log.StringValue("foo"), log.StringValue("bar"))`
instead of `log.String("[foo bar"])`. (#6254)
- Upgrade `go.opentelemetry.io/otel/semconv/v1.17.0` to
`go.opentelemetry.io/otel/semconv/v1.21.0` in
`go.opentelemetry.io/contrib/instrumentation/go.mongodb.org/mongo-driver/mongo/otelmongo`.
(#6272)
- Resource doesn't merge with defaults if a valid resource is configured
in `go.opentelemetry.io/contrib/config`. (#6289)

### Fixed

- Transform nil attribute values to `log.Value` zero value instead of
panicking in `go.opentelemetry.io/contrib/bridges/otellogrus`. (#6237)
- Transform nil attribute values to `log.Value` zero value instead of
panicking in `go.opentelemetry.io/contrib/bridges/otelzap`. (#6237)
- Transform nil attribute values to `log.Value` zero value instead of
`log.StringValue("<nil>")` in
`go.opentelemetry.io/contrib/bridges/otelslog`. (#6246)
- Fix `NewClientHandler` so that `rpc.client.request.*` metrics measure
requests instead of responses and `rpc.client.responses.*` metrics
measure responses instead of requests in
`go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc`.
(#6250)
- Fix issue in `go.opentelemetry.io/contrib/config` causing
`otelprom.WithResourceAsConstantLabels` configuration to not be
respected. (#6260)
- `otel.Handle` is no longer called on a successful shutdown of the
Prometheus exporter in `go.opentelemetry.io/contrib/config`. (#6299)