Add server.server_close() call to shutdown example to properly
release the port. Without this call, attempting to restart the
server on the same port results in "Address already in use" error.

Fixes prometheus#1068

Signed-off-by: Varun Chawla <varun_6april@hotmail.com>

Use base64 encoding for grouping key values containing spaces,
similar to how values with slashes are handled. This prevents
spaces from being converted to '+' signs by quote_plus().

Fixes prometheus#1064

Signed-off-by: Varun Chawla <varun_6april@hotmail.com>

* Support MultiProcessCollector in RestrictedRegistry.

This change makes it so that the RestrictedRegistry will always attempt to collect metrics from a collector for which it couldn’t find any metrics name. Although this can be used generally, this is meant to be used with MultiProcessCollector.

This changes the current behavior of the code but should be somehow safe as it enables filtering in case where it was not working previously. If this is an issue, an alternative approach with an explicit flag could be used (set either in the MultiProcessCollector or in the registry).

The intent here is to allow collecting a subset of metrics from production fastapi servers (running in multiprocess mode). So not having to change the library usage in these servers is advantageous to have filtering work out-of-the-box with this change.

Signed-off-by: Mathias Kende <mathias.kende@mistral.ai>

* Make the new support for collectors without names be explicit.

This adds a parameters to the constructor of CollectorRegistry to allow that new behavior rather than make it be the default.

Signed-off-by: Mathias Kende <mathias.kende@mistral.ai>

* Fix comments

Signed-off-by: Mathias Kende <mathias.kende@mistral.ai>

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Signed-off-by: Mathias Kende <mathias.kende@mistral.ai>