Fix connector validation based on usage in pipelines

[djaglowski]

Alternate to #8003

Fixes ##7892

Validation of connectors was too aggressive such that a connector that was used in any combination of unsupported roles would fail. Instead, validation should pass as long as each use of the connector has a supported corresponding use.

For example, the forward connector may forward traces and metrics at the same time. Previously, validation would fail because it detected that traces->metrics and metrics->traces were possible connections. Now it will pass as long as there is a supported connection type for each pipeline in which the connector is used.

[djaglowski]

Validation of connectors is currently split between otelcol and service/internal/graph packages, roughly as follows:

otelcol: Obvious problems that can be reasoned about just be looking at the config.

• Invalid connector type
• Connector not used as both exporter and receiver

service/internal/graph: Less obvious problems that basically require building the graph / components.

• Connector introduces cycle into graph
• Connector config invalid

The validation added in this PR could arguably be added to either package, as it seems to fall somewhere in between, but having tried adding it to otelcol in #8003, I believe this is a better solution. I also pulled the "Connector not used as both exporter and receiver" logic from otelcol into this solution because it is very closely related.

[codecov]

Codecov Report

Patch coverage: 98.68% and project coverage change: -0.10 ⚠️

Comparison is base (ec033ca) 90.90% compared to head (9fc673b) 90.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8004      +/-   ##
==========================================
- Coverage   90.90%   90.80%   -0.10%     
==========================================
  Files         300      300              
  Lines       15074    15146      +72     
==========================================
+ Hits        13703    13754      +51     
- Misses       1097     1110      +13     
- Partials      274      282       +8     

Impacted Files	Coverage Δ
otelcol/config.go	100.00% <ø> (ø)
...rvice/internal/testcomponents/example_connector.go	100.00% <ø> (ø)
service/internal/graph/graph.go	98.66% <98.68%> (-0.03%)	⬇️

... and 3 files with indirect coverage changes

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[mx-psi]

LGTM, one last thought that we don't need to address here:

Validation of connectors is currently split between otelcol and service/internal/graph packages

I am wondering if everything should be on service/internal/graph; if we are to eventually support #4970/#6888 it seems like the best solution is to expose the graph package, in which case all validation should happen there.

We don't need to solve this on this PR necessarily, since moving the validation around is not a breaking change.

[djaglowski]

@mx-psi, thanks for the reviews.

w/r/t where validation is located, I think this PR effectively consolidates it into the graph package. The only exception now is the very high level checks like "is this a valid type of connector", which we still do in otelcol along with the other components.

[mx-psi]

which we still do in otelcol along with the other components.

alright, sounds like it's something we should solve independently then :)