From bowtie2 manual for the log:

--met-stderr <path> Write bowtie2 metrics to the "standard error" ("stderr") filehandle. This is not mutually exclusive with --met-file. Having alignment metric can be useful for debugging certain problems, especially performance issues. See also: --met. Default: metrics disabled.

And just pipe the output to samtools, eg:

bowtie2 --met-stderr metrics.txt -q -x index -1 r1.fastq.gz -2 r2.fastq.gz | samtools sort -o sorted.bam