Print out response body when a 500 Internal Server Error is received from the pushgateway

I accidentally sent malformed data to a push gateway and got back a 500 Internal Server Error. The body of the response had information about the malformed data but all that was printed was the fact that I got a 500 Internal Server Error.

Example code:

import prometheus_client

reg = prometheus_client.CollectorRegistry()
c1 = prometheus_client.Counter("foo", "bar", labelnames=["baz", "stuff"], registry=reg)
c1.labels({"baz": 1, "stuff": 2}).inc(1)
c2 = prometheus_client.Counter("foo", "bar", labelnames=["baz", "stuff"], registry=reg)
c2.labels({"baz": 3, "stuff": 4}).inc(2)
prometheus_client.push_to_gateway('localhost:8000', job='example', registry=reg)

Output:

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    prometheus_client.push_to_gateway('localhost:8000', job='example', registry=reg)
  File "/usr/local/lib/python2.7/dist-packages/prometheus_client/exposition.py", line 110, in push_to_gateway
    _use_gateway('PUT', gateway, job, registry, grouping_key, timeout)
  File "/usr/local/lib/python2.7/dist-packages/prometheus_client/exposition.py", line 144, in _use_gateway
    resp = build_opener(HTTPHandler).open(request, timeout=timeout)
  File "/usr/lib/python2.7/urllib2.py", line 437, in open
    response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 550, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 475, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 558, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 500: Internal Server Error

从 prometheus_client/exposition.py 中的 push_to_gateway 和 _use_gateway 开始，使用 malformed-data 示例重现 HTTP 500 响应。检查当前如何呈现响应错误，并验证 push 失败包含响应正文，同时保留现有的错误详细信息。