support HTTPS/TLS. (#946)

* support HTTPS/TLS.
* Handling Andy's review comments
1.A better description of what's new.
2.Modify the parameter order of the start_wsgi_server method to maintain forward compatibility.
3.Enable authentication mode when safe.
* revert tls_auth_handler func.
* use "None" indicate "not provided".
* TLS: Improved error handling; Documented capath
* Update parameter names to make clear they are used to verify client certificates.
* - Rename the `insecure_skip_verify` parameter to `client_auth_required`.
- Update `_get_ssl_ctx` func, default not to set `ssl_ctx.verify_mode=ssl.CERT_REQUIRED`.
- Update the description of default in README.md.

Signed-off-by: kareza <kareza@qq.com>

---------

Signed-off-by: kareza <kareza@qq.com>
Co-authored-by: Andreas Maier <maiera@de.ibm.com>

Author: ethanschen

README.md

@@ -288,8 +288,8 @@ ProcessCollector(namespace='mydaemon', pid=lambda: open('/var/run/daemon.pid').r
 ### Platform Collector
 
 The client also automatically exports some metadata about Python. If using Jython,
-metadata about the JVM in use is also included. This information is available as 
-labels on the `python_info` metric. The value of the metric is 1, since it is the 
+metadata about the JVM in use is also included. This information is available as
+labels on the `python_info` metric. The value of the metric is 1, since it is the
 labels that carry information.
 
 ### Disabling Default Collector metrics
@@ -327,6 +327,27 @@ To add Prometheus exposition to an existing HTTP server, see the `MetricsHandler
 which provides a `BaseHTTPRequestHandler`. It also serves as a simple example of how
 to write a custom endpoint.
 
+By default, the prometheus client will accept only HTTP requests from Prometheus.
+To enable HTTPS, `certfile` and `keyfile` need to be provided. The certificate is
+presented to Prometheus as a server certificate during the TLS handshake, and
+the private key in the key file must belong to the public key in the certificate.
+
+When HTTPS is enabled, you can enable mutual TLS (mTLS) by setting `client_auth_required=True`
+(i.e. Prometheus is required to present a client certificate during TLS handshake) and the
+client certificate including its hostname is validated against the CA certificate chain.
+
+`client_cafile` can be used to specify a certificate file containing a CA certificate
+chain that is used to validate the client certificate. `client_capath` can be used to
+specify a certificate directory containing a CA certificate chain that is used to
+validate the client certificate. If neither of them is provided, a default CA certificate
+chain is used (see Python [ssl.SSLContext.load_default_certs()](https://docs.python.org/3/library/ssl.html#ssl.SSLContext.load_default_certs))
+
+```python
+from prometheus_client import start_http_server
+
+start_http_server(8000, certfile="server.crt", keyfile="server.key")
+```
+
 #### Twisted
 
 To use prometheus with [twisted](https://twistedmatrix.com/), there is `MetricsResource` which exposes metrics as a twisted resource.
@@ -393,7 +414,7 @@ Such an application can be useful when integrating Prometheus metrics with ASGI
 apps.
 
 By default, the WSGI application will respect `Accept-Encoding:gzip` headers used by Prometheus
-and compress the response if such a header is present. This behaviour can be disabled by passing 
+and compress the response if such a header is present. This behaviour can be disabled by passing
 `disable_compression=True` when creating the app, like this:
 
 ```python

prometheus_client/exposition.py

@@ -159,7 +159,58 @@ def _get_best_family(address, port):
     return family, sockaddr[0]
 
 
-def start_wsgi_server(port: int, addr: str = '0.0.0.0', registry: CollectorRegistry = REGISTRY) -> None:
+def _get_ssl_ctx(
+        certfile: str,
+        keyfile: str,
+        protocol: int,
+        cafile: Optional[str] = None,
+        capath: Optional[str] = None,
+        client_auth_required: bool = False,
+) -> ssl.SSLContext:
+    """Load context supports SSL."""
+    ssl_cxt = ssl.SSLContext(protocol=protocol)
+
+    if cafile is not None or capath is not None:
+        try:
+            ssl_cxt.load_verify_locations(cafile, capath)
+        except IOError as exc:
+            exc_type = type(exc)
+            msg = str(exc)
+            raise exc_type(f"Cannot load CA certificate chain from file "
+                           f"{cafile!r} or directory {capath!r}: {msg}")
+    else:
+        try:
+            ssl_cxt.load_default_certs(purpose=ssl.Purpose.CLIENT_AUTH)
+        except IOError as exc:
+            exc_type = type(exc)
+            msg = str(exc)
+            raise exc_type(f"Cannot load default CA certificate chain: {msg}")
+
+    if client_auth_required:
+        ssl_cxt.verify_mode = ssl.CERT_REQUIRED
+
+    try:
+        ssl_cxt.load_cert_chain(certfile=certfile, keyfile=keyfile)
+    except IOError as exc:
+        exc_type = type(exc)
+        msg = str(exc)
+        raise exc_type(f"Cannot load server certificate file {certfile!r} or "
+                       f"its private key file {keyfile!r}: {msg}")
+
+    return ssl_cxt
+
+
+def start_wsgi_server(
+        port: int,
+        addr: str = '0.0.0.0',
+        registry: CollectorRegistry = REGISTRY,
+        certfile: Optional[str] = None,
+        keyfile: Optional[str] = None,
+        client_cafile: Optional[str] = None,
+        client_capath: Optional[str] = None,
+        protocol: int = ssl.PROTOCOL_TLS_SERVER,
+        client_auth_required: bool = False,
+) -> None:
     """Starts a WSGI server for prometheus metrics as a daemon thread."""
 
     class TmpServer(ThreadingWSGIServer):
@@ -168,6 +219,9 @@ class TmpServer(ThreadingWSGIServer):
     TmpServer.address_family, addr = _get_best_family(addr, port)
     app = make_wsgi_app(registry)
     httpd = make_server(addr, port, app, TmpServer, handler_class=_SilentHandler)
+    if certfile and keyfile:
+        context = _get_ssl_ctx(certfile, keyfile, protocol, client_cafile, client_capath, client_auth_required)
+        httpd.socket = context.wrap_socket(httpd.socket, server_side=True)
     t = threading.Thread(target=httpd.serve_forever)
     t.daemon = True
     t.start()
@@ -407,7 +461,7 @@ def tls_auth_handler(
     The default protocol (ssl.PROTOCOL_TLS_CLIENT) will also enable
     ssl.CERT_REQUIRED and SSLContext.check_hostname by default. This can be
     disabled by setting insecure_skip_verify to True.
-    
+
     Both this handler and the TLS feature on pushgateay are experimental."""
     context = ssl.SSLContext(protocol=protocol)
     if cafile is not None: