application/openmetrics-text encoding cannot be selected in asgi app using the Accept header

[jezekra1]

The openmetrics format cannot be selected using the Accept: application/openmetrics-text header, because there is the extra header key in the beginning in asgi.py:make_asgi_app which I believe is incorrect. Compare with the accept_encoding_header just below.
https://github.com/prometheus/client_python/blob/master/prometheus_client/asgi.py#L15:

         # Accept header key here:
        accept_header = "Accept: " + ",".join([
            value.decode("utf8") for (name, value) in scope.get('headers')
            if name.decode("utf8").lower() == 'accept'
        ])
        # No header key here:
        accept_encoding_header = ",".join([
            value.decode("utf8") for (name, value) in scope.get('headers')
            if name.decode("utf8").lower() == 'accept-encoding'
        ])

The condition in exposition.py is thus comparing the full header:

accepted.split(';')[0].strip() results to Accept: application/openmetrics-text.

def choose_encoder(accept_header: str) -> Tuple[Callable[[CollectorRegistry], bytes], str]:
    accept_header = accept_header or ''
    for accepted in accept_header.split(','):
        if accepted.split(';')[0].strip() == 'application/openmetrics-text':
            return (openmetrics.generate_latest,
                    openmetrics.CONTENT_TYPE_LATEST)
    return generate_latest, CONTENT_TYPE_LATEST

[csmarchbanks]

Good find! Are you interested in creating a PR?

[csmarchbanks]

Fixed in #864