Update write endpoint

[carrieedwards]

This PR updates the write endpoint that is registered as a route in the proxy. This endpoint needs to be updated in order to allow for applications such as the InfluxDB client API to push directly to the proxy. The expected endpoint for the InfluxDB client is "/api/v2/write".

[grafanabot]

Go coverage report:

Click to expand.

File	%
github.com/grafana/influx2cortex/pkg/errorx/errors.go	67.3%
github.com/grafana/influx2cortex/pkg/errorx/http_translator.go	100.0%
github.com/grafana/influx2cortex/pkg/influx/api.go	100.0%
github.com/grafana/influx2cortex/pkg/influx/errors.go	100.0%
github.com/grafana/influx2cortex/pkg/influx/parser.go	84.1%
github.com/grafana/influx2cortex/pkg/influx/proxy.go	63.4%
github.com/grafana/influx2cortex/pkg/influx/recorder.go	91.7%
github.com/grafana/influx2cortex/pkg/remotewrite/client.go	69.1%
github.com/grafana/influx2cortex/pkg/remotewrite/measured_client.go	88.9%
github.com/grafana/influx2cortex/pkg/remotewrite/recorder.go	100.0%
github.com/grafana/influx2cortex/pkg/route/registerer.go	100.0%
github.com/grafana/influx2cortex/pkg/server/middleware/http_auth.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/http_fake_auth.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/instrument.go	6.7%
github.com/grafana/influx2cortex/pkg/server/middleware/logging.go	69.7%
github.com/grafana/influx2cortex/pkg/server/middleware/middleware.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/request_limits.go	81.8%
github.com/grafana/influx2cortex/pkg/server/middleware/response.go	57.1%
github.com/grafana/influx2cortex/pkg/server/middleware/route_matcher.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/tracer.go	13.0%
github.com/grafana/influx2cortex/pkg/server/server.go	83.3%
github.com/grafana/influx2cortex/pkg/tenant/resolver.go	75.6%
github.com/grafana/influx2cortex/pkg/tenant/tenant.go	91.7%
github.com/grafana/influx2cortex/pkg/util/bytereplacer/byte_replacer.go	100.0%
github.com/grafana/influx2cortex/pkg/util/extract_forwarded.go	66.7%
github.com/grafana/influx2cortex/pkg/util/http.go	68.3%
github.com/grafana/influx2cortex/pkg/util/log/experimental.go	0.0%
github.com/grafana/influx2cortex/pkg/util/log/log.go	0.0%
github.com/grafana/influx2cortex/pkg/util/log/rate_limit.go	90.0%
github.com/grafana/influx2cortex/pkg/util/log/wrappers.go	0.0%
github.com/grafana/influx2cortex/pkg/util/push/push.go	60.0%
total	65.3%

Go lint report:

No issues found. 😎