This patch makes sp.GetRecording() return nil if the span was not
recording. This is already what the documentation on GetRecording() was
implying, but the actual behavior was different: a non-recording span
(that's not a no-op span) would return a non-nil recording consisting
only of the the span's metadata. This behavior is not necessarily
unreasonable, but it turns out it's a bit dangerous from a performance
perspective - there's a cost (e.g. some allocations) for generating this
recording, and it was too easy for unsuspecting callers to pay that when
they didn't want to. In particular, DistSQL was indiscriminently calling
GetRecording() (through its GetTraceData* utilities), and paying this
measurable cost unintentionally.

Eliminating this cost is particularly important when the Tracer is
configured in the "active spans registry" tracing mode; when not in that
mode, all the spans would be no-ops and the GetRecording calls were
shortcircuited anyway. This is seen in BenchmarkTracing//trace=on,
particularly on "3node/scan" which was the benchmark with the bigest
difference between tracing on and off.

name                              old time/op    new time/op    delta
Tracing/1node/scan/trace=on-32       214µs ± 4%     206µs ± 5%    ~     (p=0.222 n=5+5)
Tracing/1node/insert/trace=on-32     338µs ± 5%     334µs ± 4%    ~     (p=0.690 n=5+5)
Tracing/3node/scan/trace=on-32       515µs ± 3%     484µs ± 3%  -6.11%  (p=0.008 n=5+5)
Tracing/3node/insert/trace=on-32     635µs ± 1%     627µs ± 1%    ~     (p=0.095 n=5+5)

name                              old alloc/op   new alloc/op   delta
Tracing/1node/scan/trace=on-32      27.2kB ± 2%    25.6kB ± 2%  -5.94%  (p=0.008 n=5+5)
Tracing/1node/insert/trace=on-32    45.2kB ± 2%    45.5kB ± 2%    ~     (p=0.690 n=5+5)
Tracing/3node/scan/trace=on-32      83.2kB ± 3%    79.7kB ± 5%    ~     (p=0.095 n=5+5)
Tracing/3node/insert/trace=on-32     102kB ± 2%     103kB ± 1%    ~     (p=0.548 n=5+5)

name                              old allocs/op  new allocs/op  delta
Tracing/1node/scan/trace=on-32         242 ± 1%       229 ± 0%  -5.37%  (p=0.000 n=5+4)
Tracing/1node/insert/trace=on-32       338 ± 1%       339 ± 1%    ~     (p=0.563 n=5+5)
Tracing/3node/scan/trace=on-32         819 ± 1%       788 ± 6%    ~     (p=0.135 n=5+5)
Tracing/3node/insert/trace=on-32       871 ± 0%       880 ± 0%  +0.99%  (p=0.016 n=4+5)

This patch also introduces Span.GetConfiguredRecording(), as a simpler
flavor of Span.GetRecording(<rec type>). The latter asks the caller for
a recording type (for example, one can ask for a Structured recording on
a span that's recording verbosely). The need for this flexibility is
rare, so `GetConfiguredRecording()` uses the span's current recording
mode.

Release note: None