Add support for Prometheus OpenMetrics about asab

[nicolatjensen]

I've been using Prometheus for a while and I only needed to add a service like this:

import re
import sys
import traceback

import aiohttp.web
import asab
import prometheus_client


class Metrics(asab.Service):
    def __init__(self, app: asab.Application, service_name=__name__):
        super().__init__(app, service_name)
        self._charset_regex = re.compile(r'(^|;)(\s*charset\s*=\s*([^;\s]+)\s*;*)', flags=re.IGNORECASE)

    async def initialize(self, app: asab.Application):
        await super().initialize(app)
        web_service = app.get_service('asab.WebService')
        web_service.WebApp.router.add_get('/metrics', self._metrics)

    async def finalize(self, app: asab.Application):
        await super().finalize(app)

    def _metrics(self, request: aiohttp.web.Request) -> aiohttp.web.Response:
        try:
            accept_header = request.headers.get('Accept')
            encoder, content_type = prometheus_client.exposition.choose_encoder(accept_header)
            # separate the charset from the content-type
            matches = self._charset_regex.findall(content_type)
            if matches:
                charset = matches[-1][2]
            else:
                charset = None
            content_type = self._charset_regex.sub(r'\1', content_type)
            return aiohttp.web.Response(
                body=encoder(prometheus_client.REGISTRY), content_type=content_type, charset=charset)
        except Exception:
            exc_type, exc_value, exc_traceback = sys.exc_info()
            error = ''.join(traceback.format_exception_only(exc_type, exc_value)).strip()
            raise aiohttp.web.HTTPInternalServerError(reason=error.replace('\n', ' # '))

[ateska]

Yes - in order to explore this a bit more, we need to integrate prometheus_client with asab.metrics