Change response time histogram bucket sizes

[ulope]

Description:

The synapse_http_server_response_time_seconds histogram metric uses the default bucket sizes of .005, .01, .025, .05, .075, .1, .25, .5, .75, 1.0, 2.5, 5.0, 7.5, 10.0, _INF.

Considering that this also contains the sync endpoint which is expected to be in the 30s range quite often (and sometimes more for complex responses) it would be useful to change the bucket sizes to better reflect the actual values.

Another option would be to split out the sync endpoint response time into its own metric.