contrib/google.golang.org/grpc: Reduce allocs in interceptor

[evanj]

finishWithError needs to allocate a []tracer.FinishOption slice to
pass arguments to span.Finish. However, those arguments are only used
if err != nil. Only allocate the slice if it is used. Allocate the
exactly right size of slice. This appears to remove 3 allocations in
the "no error" case, and 1 allocation if the NoDebugStack() option is
specified. In the case of an error, there is no difference. This may
also make a small difference in the run time. The benchstat results
from my Mac laptop are below.

name                                                         old time/op    new time/op    delta
UnaryServerInterceptor/ok_no_metadata-8                        5.33µs ± 8%    4.73µs ± 5%  -11.27%  (p=0.000 n=9+9)
UnaryServerInterceptor/ok_with_metadata_no_parent-8            6.44µs ± 7%    5.71µs ±13%  -11.41%  (p=0.001 n=9+10)
UnaryServerInterceptor/ok_with_metadata_with_parent-8          5.52µs ± 9%    5.03µs ± 8%   -8.94%  (p=0.002 n=10+9)
UnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8    5.61µs ±10%    4.94µs ± 3%  -11.89%  (p=0.000 n=10+10)
UnaryServerInterceptor/error_no_metadata-8                     13.3µs ±15%    11.6µs ± 1%  -12.90%  (p=0.005 n=10+7)

name                                                         old alloc/op   new alloc/op   delta
UnaryServerInterceptor/ok_no_metadata-8                        4.94kB ± 0%    4.85kB ± 0%   -1.87%  (p=0.000 n=9+7)
UnaryServerInterceptor/ok_with_metadata_no_parent-8            5.57kB ± 1%    5.46kB ± 0%   -1.91%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_with_metadata_with_parent-8          4.66kB ± 0%    4.57kB ± 0%   -2.05%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8    5.14kB ± 0%    5.05kB ± 1%   -1.78%  (p=0.000 n=10+10)
UnaryServerInterceptor/error_no_metadata-8                     11.3kB ± 0%    11.3kB ± 0%     ~     (p=0.066 n=10+8)

name                                                         old allocs/op  new allocs/op  delta
UnaryServerInterceptor/ok_no_metadata-8                          52.0 ± 0%      49.0 ± 0%   -5.77%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_with_metadata_no_parent-8              57.0 ± 0%      54.0 ± 0%   -5.26%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_with_metadata_with_parent-8            48.0 ± 0%      45.0 ± 0%   -6.25%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8      54.0 ± 0%      51.0 ± 0%   -5.56%  (p=0.000 n=10+10)
UnaryServerInterceptor/error_no_metadata-8                       69.0 ± 0%      69.0 ± 0%     ~     (all equal)